Unfortunately staying is the worse choice for both  you and DS and is a naive perspective if what you truly want is what is best for your DS..... and for yourself.  Kids should have examples of strong, confident parents in mutually respectful loving relationship. If you stay, the odds of DS having that are about zero.

If you end it now and move on for your own happiness, you will be able to model being a confident mom for your DS.  You will also have the opportunity to have healthy adult relationships and in doing that, have every opportunity to show your DS a close, mutually respectful equity life partnership with a quality partner.

I am a "childless"SParent in my marriage.  My DW and I met and started dating when SS-28 was 15mos old.  We married the week before he turned 2yo.  All he has memory of is his mom and I being together, making a life together, and raising him together.  We will celebrate our 27th anniversary this year.  His mom moved on to avoid putting herself and SS through the relationship train wreck of staying with the SpermIdiot.  He cheated on my single teen mom bride and that was the end of her tolerating his crap. She left the State for university taking SS with her and she never looked back.

Your DH is cheating on the relationship he has with you with his own daughter.  Minimizing the exposure that you suffer and your own son will suffer due to that crap, IMHO, has to be a commitment you owe to yourself and your DS.

Kids should see their parents happy, and you should be happy.  

Make that happen for yourself and your DS.

End the cycle of sacrificing yourself on the alter of SParental martyrdom to an emotionally incestuous relationship between your hopefully STBXDH and his prior  failed family relationship daughter and demonstrate confidence and happiness for your kid.  Living well is the best revenge.  Leave DH to wallow in the toxicity he has chosen to create with his mini-wife.

My SS would regularly comment about his Bio-Dad's relationship train wrecks when he would return from SpermLand visitations.  He would also comment about the crappy relationship between his SpermGrandHag and his SpermGrandPa who stayed married for their kids  As he got older the foundation of a strong and respectful relationship and equity life partnership that his mom and I demonstrated was a notable antidote to the multigenerational relationship toxicity he experienced and observed his SpermClan living and watched his three younger all out of wedlock half sibs by two other baby mammas being victimized by.   

My SS is a wonderful man living a great life and has learned how to love himself healthily and be a good mate to his partner.

Your DS deserves to see his mom happy.

Keep that in mind.

Staying together "for the the kids" is why my SS's SpermClan is so screwed up, including not only the SpermIdiot and his sister, but also my SS's three younger SpermIdiot spawned half sibs who were raised by the SpermGrandHag and the serially adulterous mini-wife loving SpermGrandPa.

SS asked me to adopt him when he was 22.  We made that happen.  He is living a great life, is a successful and happy young man, is successful in his profession, a man of standing in his community, and is a good partner to his mate.  I am proud that much of that is due to the examples his mom and I set while making our lives together.

Good luck.  Go for happy rather than martyrdom.  Both you and your DS will be far better off. 

It's easier on children if you leave when they are young. Then they grow up not knowing any different. It's much harder for them to adjust to divorce as they get older.

I also agree with Rags you will have an opportunity to possibly meet someone great and will have the opportunity to model what a healthy relationship looks like for your child.

I wanted the best for my son and ME. Was everything I was going through worth the stress? No! 
 

leabing my OH was the best decision I made for myself. My son is one soon. He won't see an unhappy mother and will understand the sacrifices I made for him and our life together. Dealing with it sooner rather then later made me feel so strong and capable of going through anything, it's likely things won't get better anytime soon, but if you have the strength now - do whats best for you. I've been parenting completely by myself for the past 3 months and while it's been hard... it's been so rewarding because I know my child will have the right values when he's older and will know what I did was the best thing I did for him at the time. 

Mini wifes never go away. They will pull their shit unless big daddio stops it. Most big daddys love their mini wife, makes them feel so needed. Mine left me, so he ended it.BUT I have never been happier and peaceful. I have no bios, so no guilt there. A hole pulled a switcharoo and didnt want more kids after we married. Having one perfect mini wife was all he wanted.

Better to leave when your DS is young. Also does he favor his mini wife over DS. That would be my concern.