What the actual fuck.
I'm constantly surprised just how many absolutely disgusting specimens there are out there. OP - he's foul.
You seem pretty sure that there really isn't a future, I'd totally agree and am sure there's a lot more you could tell us. So, to move on to contact - I think a lot of your fears here are unfounded.
Firstly, the sooner you leave, the better. No, it ISN'T true that a court will put a father's contact above the needs of a bf baby. Not at all. As long as he HAS contact - and yes, that might end up being for a small amount of time each day even - then no, your DS won't be taken from you to be with him for significant periods of time until he is a lot older. No overnights for a long time, for sure. Don't stay with him because you are afraid your child will be taken from you - he won't. Get the ball rolling by talking to your midwife or health visitor about how he is, INCLUDING that scene (horribly embarrassing, but you don't need to go into too many details - just let them know that he is harassing you sexually including times when the baby is breastfeeding and he can see your breasts etc., that he is groping you). Lay a paper trail.
Secondly, it's interesting that you immediately say that he would want access because of his family and keeping up appearances. Not because he loves his son! As others have said, this is the type that loses interest pretty quickly. Make a lot of noise about 'his rights' - but when faced with the reality of every Saturday night at home with a crying baby/toddler while you make it clear that you think it's GREAT that he has time alone with his son and make sure he knows that you'll be using that time to socialise... wait and see how quickly those visits dry up! Do you live near his parents/sister? If not, and he can't offload the care on a ragular basis, this goes double.
As for the divorce and SIL - well, the quicker you do it, the more time you have to come back from any financial hit and get your life back on track. Shit-hot solicitor or not, you won't come away with nothing.
Good luck.