sleep issues based around anxiety

Hi
My son is 7.5 and diagnosed ASD with GDD, we are experiencing major sleep issues based around anxiety. Hugo does not settle well and is often hysterical when left on his own in his room, he won't stay there and wanders back and forth flapping and repeating 'cuddle up' continuously, he has a reasonable bedtime routine, bath, books, sleep all done in his own room. He is taking melatonin which has helped a little with the initial settling (but not every night) but once he has fallen asleep on his own he only sleeps for 2-3 hours and jerks a lot in his sleep, then he wakes in a blind panic and refuses to sleep alone.....I've tried returning him to bed quietly but he becomes hysterical and the only way I can get him to sleep is to let him go to my bed with me, where he then wakes every hour or so in a panic and needs reassurance to settle again, I am exhausted and so is he, any advise welcome Donna x