OP, I know this is hard, but I tend to think this is your son's bereavement, not yours. that's a hard phrase to start with - it is meant kindly. I have been in the exact same situation and I had to learn to take second place. Admittedly my kids were slightly older than your ds but ex's family hated me - and tbh I had to take a back seat and let my kids grieve with the people who loved their dad. I didn't belong to that circle iyswim. It seems that your ds is being excluded by ex's family bcs of their hatred for you - can you let him contact ex's family directly and go to the funeral with them? it's a risk, for sure, as they hate you - but to go to the funeral (and also the wake) is essential for your ds to come to terms with his dad's death imo. You could maybe have a word with them that, yes, they hate you and bcs of that you are proposing ds goes on his own to the funeral with them (ie not with you) but ask that, please, to not put you down in front of him - he has lost one parent and can't 'lose' the other.
Ask them to do it for him and to put aside their feelings about you just for the day? If you can find it in you, offer your condolences to them? I'm only thinking to pour oil on a very painful situation. They have lost a brother/son/etc and will not be thinking straight, feelings running extremely high - I read somewhere that grief is the only time you can go legitimately mad. Their bullying of you has nothing to do with ds, so step aside? I appreciate this is hard to do.
My boys didn't want to talk about losing their dad for years. I got everything in place - books, counselling etc - but they absolutely did NOT want to know. dd reacted very differently to the boys, grief apparent almost instantly, but ds1 told no-one but his best friend as he didn't want to be known as the boy whose dad died. Both boys were calm and dignified during the funeral etc but to this day I have not seen either of them cry about their dad. I had to butt out and leave them to it, even though it was tough for me - they specificially asked me to back off. Boys behave in a way that looks suspiciously like a stiff upper lip but is, imo, just male (I am generalising, but this is what I found with my boys).
As for your friends, it could be that they have picked up some strong 'piss off' vibes from your boy and have acted accordingly. They also may have approached him privately when you weren't there. I wonder if your own bereavement (re your mum - I am so sorry it was such a tough bereavement) is clouding how you are viewing your son's bereavement and you are panicking, wanting him to do it 'properly' in those early weeks/months. I did this ie panicked but had to accept that they will do it their way and I can only be on hand should the need arise.
Hope I haven't offended you OP. I do know first hand how tough this situation is.