Hey M. (my name is M., too!)
A friend of mine went through something similar with her eldest boy. Her ex was emotionally abusive, too. She divorced him when her boys were 4 and 6. Her ex wanted almost no contact with the boys, which she was glad about.
However, when her eldest turned about 15, he started to really act out and get into trouble. When his mom stuck by her rules, he said he wanted to go live with his Dad. His father surprisingly agreed to this. It was very, very tough for my friend. Anyway, two weeks after he left to go live with his dad, he called his mother crying, begging him to let him come home. His father did not try to hit him or anything, but he spent almost no time with the boy. He also said some very unkind things to him.
She told him he could come home as long as he agreed to follow her rules (no drugs, no drinking, go to school, stick to the curfew). He agreed, he came home and, all things considered, did really well the rest of high school.
Your situation is not the same, of course. To begin with, your son is only 13 - 15 is young, but 13 seems really young to me. Your son also has some learning disabilities, which would only complicate the whole thing.
My only point is, try to give your son a dose of reality with his dad.
The pp's advice is good: get the older children to talk to him and/or try to arrange a SHORT visit with his dad.
Maybe one of his older siblings would be willing to tolerate dad for a week, if they were told they would be going to protect their youngest brother? A long shot, I know.
Could you travel with him while he visits dad for one or two weeks - stay a hotel nearby perhaps?
I agree with you about not telling your son all the lousy things his father has done. To begin with, at age 13, he's not going to believe you. If his brothers can tell him, that would be more convincing.
If you son is absolutely convinced that this is what he wants to do, you might want to consider the idea of him going. That would totally depend upon just how abusive your ex is. Another idea: tell you son that when he turns 15 (or 16), if he still wants to live with dad, he can.
I hope this works out - M.