Your GF and the BF are parents.  You are not.  Therefore, you should not be taking on parenting duties to the point you are not working enough hours and only have a tiny fraction for the money you need.  Why in the world does this sound like a good idea to you?

You could be left with nothing - no savings and no friends at the end of this.  Plus read around on the boards here - skid will never appreciate sacrifices you make.

GF needs to figure out care for her child.  It should not mean you giving up an income.  
 

Id also be very leery of anyone wanting to take away parental rights unless it is an abuse situation. If bio dad was good enough to make a baby with, and isn't abusing the child, then cutting out bio dad to insert you in the role of dad is very concerning.

You are putting yourself in a situation that doesn't seem to either fit who you are as a person and one where you are making financial sacrifices.  I'd take a step way back and ask yoirself why.  Love is t enough in step situations.  You need to take financial care of yourself first not try to solve your GFs problems which will only lead to resentment.

I'm totally confused. She works full-time during the day? Why do you need to change your schedule from overnights? Can't she get him ready in the morning for school/day care and you are there for a few hours at the end of the day before she gets home?

At any rate, NO - don't do it. He's not your kid and not your responsibility, and I promise you will resent not only giving up your preferred hours, but being the primary caregiver.  What would she do if she were single? Day care, Dad, Grandma ... she needs to use those resources.

As for the father, leave well enough alone. If he's not coming around anyway, she doesn't need to go for full custody.

You're kidding, right? Look, if SHE wants to go for full custody, the situation is entirely on her, not you. She pays the court costs, not you. She is there for her son, not you. Do not jeopardize your life for her whims. 

If the father is taking care of his son 3 days a week, he isn't a deadbeat. It seems to me that your gf wants to stick it to her ex no matter what the cost and you would be collateral damage.. YOU are not the one being selfish. 

When she can't be there for her son  at the drop of a hat but you will have to? A parent shouldn't try and take full custody if they have to rely on everyone else to care for their kid and the fathers the slightest bit willing to have a relationship. 

If it wasn't for the age difference I'd think my ss stepdad was writing this... Mummy cutting real daddy out the picture as soon as she's got a bf as hired help. Has the daddy always been a waste of space or has she pushed him out as you suit her plans and 'new family unit' better? 

Do NOT under any circumstance change your job/work hours/pay to accommodate your GF's desire for full custody. You are neither a parent nor a nanny. If anyone should be changing hours/jobs, it should be her. Alternatively, she can hire help for her child (and pay for it). If she can't handle a change on her own, she shouldn't make it. You will regret this if you agree to it.

My mother always taught me to let a man work as much as he wants to and do not interfere with his work, ever. Your gf is disrespecting you by asking a man to work part time and raise her child. My dh works 7 days a week and I would not ever think of asking him to not work so much. I am very old fashioned but I don't think that you should adjust your life at all to be the caregiver of this child. I'm cringing just thinking about this. You need to have a full time job to provide for yourself and your future family. IMHO that is when a man is truly fulfilled. Like I said, I'm very old fashioned so you may not agree with my thoughts. Good luck! 

There are child care options around all of this. Mom can drop the child off at school early on her way to work and he can go to after school club until she finishes work. Or she can find a local child care provider or an other parent who can look after the boy before and after school for a fee. Or maybe there is a parent who can walk the boy to the local library where he can do his homework until mom can collect him. Lots of parents work full time and find ways to work around it. You do not need to change your routines, income, social life etc so that she can avoid paying for some child care. 

Helping out once in a while is nice and great and hopefully your gf is greatful. But making permenant changes to your life that will be detrimental to you is asking A LOT and is totally not something a reasonable person would expect from their partner. 

I would not do this. For one, I know I would grow resentful of this situation over time. It is not my calling to raise another person's child, I know this. Two, it impacts my life too much when it's not my responsibility. Sure, I'll help out but to rely on me daily for childcare needs is beyond the scope of what I signed up for. Three, if this impacts your paycheque, will your SO be making up for it by paying you for childcare? What if you split? Will you have enough savings to handle any emergencies? 

This impacts you both short and long term. I would really mull this one over before I made a decision. 

I'm confused. Is Dad taking his custody time? If so, a court will rarely take time away. Usually it requires Severe neglect not just lazy parenting. 

Why would you need to change so much of your life? 

I am on team don't change your work hours.   If your SO wants a change in custody, she has to be the one to facilitate it and support it financially with zero impact to you.

While I am definitely one to support true equity life partnerships including both partners being equity parents to any children in the relationship home, a significant change in custody as the sole decision of only one partner is not what I would consider a joint decision.  If SO goes for full custody, the cost of that legal action, the costs of added Skid time in  your home, and the care and feeding of that skin while in your home is on her.

Good luck.