Is there any way you can speak to your DM about this OP? Just thinking, as it sounds as if thats mostly where he is when he's not with you or at school.
His dad's died, thats a bereavement for you too. Your head's probably all over the place, a bit hard to behave in the "correct" way and do the "correct" thing, I guess
Please do go to your GP - you, as well as your little boy, need advice help and encouragement. Is there anyone at all who can support you in RL ie trusted friend or family member, who could accompany you to GP, make it easier for you to get there, and then sit in waiting room until your appointment is finished?
It needs to be sorted for both your sake. The school are ok now but if his behaviour continues in this way, they won't be ok about it for long and that will leave you with so much more to deal with, not least if parents of children he is speaking too in such a way get wind of it
You need help your son needs help. You're both struggling. You've managed to speak on here so go and say same to GP, take steps towards getting this load off your mind, and you and your son feeling better.