Bitting/hitting & shoving, PLEASE HELP

There’s a nearly 2.5yo in our mothers group who is a bully to put it nicely! It’s been happening for at least 6mo now. All his mum does is say sorry john (not his name) and then puts him in the corner. Today he was at his worst. It was their turn to host at their house and he bit 5 kids that I saw, shoved, kicked you name it he did it. It’s one thing to see (& intervene) when he does it to another child but when it happens to mine it another story! I had to do everything to hold back from wanting to throttle him (it sound awful I know) but this morning he shoved & sat my son because my son was playing with a toy that he wanted and he wouldn’t give it to him. Then they were all inside an my son was looking through a book and ‘john’ walked over & went to grab it off him and again wouldn’t let go so ‘john’ kicked my son twice in the head while he was sitting down & then thirdly because my son would give him a truck he was playing with he took a bite of him. The ‘attacking’ is always a result of another child having something that ‘john’ wants and is not given it. His mum tells him no ‘john’ don’t bite and if he wants something to say ta but this is not working as ‘john’ thinks when he says ta that he should be given what he wants that second. Whenever ‘john’ is somewhere we go (& this goes for A LOT of the other kids) I have to look over him like a hawk to make sure ‘john’ isn’t attacking him or other kids. I am soo upset by what happened to our son this morning