This has turned a bit long and garbled sorry. 
I had a long running support thread last year here (don't read it all, it's meag long, it's just a reminder for those who were on it) about leaving my emotionally abusive ex and had support from MN, RL friends and Women's Aid. I left, me and DS much happier. 
Anyway since November DS has been going twice a week to see his dad but hasn't been that fussed about going (goes though as he looks forward to seeing his eldest brother and uncle who are also there). Ex has now cut one night of his contact to just tea and since then DS has been more anxious about going and last weekend ended up coming home at nearly 11o clock as he was sobbing unconsolably for me and would not settle.
Him and ex have never really bonded closely, his dad never really bothered with him - last week I had to suggest when I dropped him off that he play a game with him FGS! 
I don't know what to do about it. I have always encouraged DS to see his dad and have actively avoided slating ex in front of DS.
DS is autistic and doesn't like change, needs a lot of reassurance, etc. Ex has never read any of DS's reports or anything (he is lazy and uninterested in anything that's not about him).
He is refusing to sleep tonight and asking to sleep at my mums instead so it's not a clingy to me thing, I think it's just that my mumis better with him and he feels more comfortable there.
Advice please 