I should tell you about me before I answer this, I may be qualified, I may not. I may be off key by miles. I have done child psychology at Uni (not a degree though), I spennt 2 years trainig as a paediatric nurse, with some time working wth children with special needs and mental health problems. I also have a gifted child who has had anxiety and depression caused by school.
It sounds to me like he's a very sad little boy and he's behaving this way out of sheer frustration. If he's unable to write and do things like the rest of his class, he's going to find this incredible frustrating. You say that everyone hates him because they just do, he's reacting to this. Everyone, adults, children, all need to feel as if they belong, as if they are wanted and loved. It's a balance, if they don't feel like this then it upsets the balance and this is when people and children start to feel that no one cares for them. This is a downward spiral and I think this is very dangerous because in the long term the effects can destroy that relationship and every relationship that this person has. It looks to me (although please remember that I do not know your family nor do I know the complete picture) like your family has excluded him to such an extent that he's unable to cope. You really need to do something about this. I would contact your GP about some family therapy. Perhapse the play therapy isn't working because this is what he wants at home, for people to play with him. If the rest of his family are not interacting with him he's probably feeling very alone. You need o talk to them all, you need to work together and make him feel included and part of the family.
With some children, if you ignore their bad behaviour they feel like you are ignoring them, which is not going to help him. He's getting worse because he's becoming more and more unhappy. Do you spend time with him alone? does he still behave like this when he has your attention and you are showing him that you care and that he's important? It's a good sign that he has friends at school, the crying is his way of showing how miserable he is. This is going to sound really silly but have you tried cuddling him and stroking his face to calm him down? I'm sorry to be so blunt, I lived in a family like this, I have worked with families like this. I may be totally wrong.