Relationship going down

I’ve been with the person I thought I loved for three years and had our son within one year together ,he just turned two, but let’s go back to when me and him split for a bit to figure things out and we ended back together , two month in being back on I’ve noticed he was being a bit weird and in April I found out he was cheating on me for two months, I had him stop this which he did 4 months later she appears again and with news that she is currently 6 months pregnant and she had just found out the week before oh and she’s having twins . I get furious and Things go down from there . He had no doubt that their his and wants to be part of those kids life which I can’t stop him from doing . And can I add in that I’m currently expecting baby #2 now too ever since she popped up saying she was , he lost all type of attention for me and my child and his son . He thinks it’s okay to sleep at her house to comfort her but yet he has his son and I at home too , they are apparently not together so I’m lost in why he has to be there when he already said he would be around the kids but why now . My son is aware of many things at his age and when he sees that his dad goes missing for a couple days and comes back out of the blue , it confuses him .. I just need some support and advice on what to do now

He’s only known her for two month until she reappeared again .

I’m a young mom , I’m 23

She just turned 19