You should write to him and be his friend, but only his friend. Do not get romantically involved. He is no position to start an emotional relationship and he won't be for some time. Even if he gets out of jail soon he should get his life straightened out before involving someone else in his mess.
There's not much you can do. Ask him to put you on his visiting list, so you can go to see him once a week. He'll be glad to have someone to visit with.
im 13 i cant go visit him i dont think