I'd feel incredibly angry. So he's not paying his Child Support, but he's giving BM "gifts" of money to use for herself?

Not sure how long you've been together, but this would be a pretty big deal for me - that's he's giving her money, and that he's lying about it. (Not telling you IS lying).

He's too enmeshed with his X for me. I'd write this one off and a lesson learned. You can't commit yourself to a man that is obviously still in some capacity involved with the X and when it comes to his finances.....well that would be a hell no for me.

If you go through with this union, keep your money separate.

I would be absolutely livid if I were you. It's bad enough that he's giving his ex money for stuff like this but the fact that he hid it from you is a huge red flag. If you decide to marry this man please keep your financies separate!

Do you share expenses or have joint finances? Is any of the money he's given her either coming from or subsidized by you in any way?  Does he have the "it's my money I can spend it how I want" attitude?

None of what he's done bodes well for your future marriage. In addition to being a bit underhanded about this and keeping it from you, he is stupid. If CS is supposed to go through the system, and he's giving it to BM outside of the system in random lump sums, those could be viewed as gifts, and he'd still owe his arrears.

I'd have a conversation about this and let him know it's unacceptable and harmful to your relationship. See how he reacts - that should let you know which direction to move in.

This is probably not the first time he has lied - or the last.  It won't get any better either.  Dump his a$$ like yesterday.

WEll, I would definitely rethink this as finances are a HUGE sticking point. Joint tax returns? They will take his back support, even if some of it is yours.

Keep finaces separated. Have a HUGE talk, about what you need from him. Rethink this engagement.

This is NOT acceptable. What his actions are saying is "your opinions do not matter in my life." He's manipulating and lying to you about what he's doing to paint a certain picture for you. He's broken down trust and now you cannot trust him with what he says he's doing with money (or you shouldn't). It would be totally reasonable for you to tell him his major unilateral decisions is a dealbreaker. But it would also be reasonable to tell him he has no business being in a relationship when he doesn't know how to act like a partner, and dump his ass.

First of all, no matter how you proceed, make sure your money and assests are 100% in your name and separate from his, and make sure he has no access. Next thing is to tell him exactly how you feel. Let him know it will not be tolerated. Let him know he should be working with you on these decisions and at least keeping you in the loop about what he's doing. 

Honestly this is a major red flag right before your marriage. Unless he corrects his behavior immediately, I would drop him and never look back.

This would be a deal breaker for me.  You should feel incredibly left out like you said, because you were!  And him lying about money by omission is NOT a good way to start any kind of relationship.  Pump the breaks on the marriage plans.

this certainly isnt a good start to your upcoming marriage.  Was there a reason he felt the need to hide this from you ?  all he is saying is "i hear what your saying , but i dont care" or hes too chicken shit to say no to his ex and too chicken shit to be upfront with you.  From my personal experience , please make sure he doesnt have anymore skeletons in the closest when it comes to the ex and kid.   at the begining I was under the impression my SO and the BM were commited co-parenters but this was not the case ,, later i found out that she was still partly living in his house coming and going back and forth between her sisters house because she lost her house from being a party girl,,,my SO was too piss weak to say NO! The long downhill run of multiple skeletons being exposed started happening ,,, what can i say , 8yrs of bullshit that i finally had to end .  Good luck and please be very wary as there really is no reason to hide that from you 

Time to to cut him off from any money that you provide and have 100% of your marital income go into an account with only your name on it. He can have his CS withheld directly from his pay checks and that is it as far as money going to BM.

Better yet, take everything and cut him loose.  

A lie of omission is no less a lie and taking money from marital resources without discussion and approval from the spouse is a write off offense. That he did this before your wedding is no less a betrayal than if you were married.  

Move on and leave this XW enmeshed and codependent ball-less wonder to wallow in the stench of his baggage with you long gone and enjoying a life without him.