Cheating partner and domestic violence

Hello, I'm in a relationship since 3.5 years. Over the years I found out more and more that my partner has been on several dating sites, texting, chatting, calling and skyping to/with other girls. He always promised to stop but never did. He seems to be very sexual, I also am but he never wants to sleep with me. Over the past 6 months it all got vworse, when I got suspicious (and I was always right) we'd argue so bad he'd hit and punch me in the head. He now asked for therapy and will get it soon. I wanted to break up several times but he breaks down crying and begging for me to stay but also doesn't change. Why does he say he loves me and wants to be with me desperately and is devastated when I say I don't know if I love him, when he clearly hates and disrespects me and can get another girl anytime? What does he still want from me? We also have a 2 year old son, he loves his Daddy and his Daddy loves him, I don't know what to do anymore.

Your abusive relationship is not good for you and is not good for your son. You are not doing your child any favors by staying with a man who abuses you physically and emotionally. To answer your question, your partner cries and wines because it works. You will stay in your place like a good dog and he will prowl the streets and mess with who ever he feels like. And since you and he have made this a little routine of yours he will most likely become less and less "good" at hiding it until it is just out in the open and a fact of everyday life for you. You need to get yourself prepared and then leave him (even if it is only temporary). He needs to know that you mean business. You haven't meant business this entire time. He can do what ever he wants and then cry to you and you will stay and if the crying wont help then he can always punch you around until you shut up about it. He is not devastated about you leaving him. If he was he wouldn't do what he is doing to have you want to leave him. He wants you to be his door matt and maybe not have to pay child support when you leave and take custody of your son. If he get therapy, great, but I think he should do it with you and him separated for a while. He needs to make amends and prove to you that he is worth having around.