Compulsory is fine if there are problems with effort of attainment and intervention is needed. But A) your DS doesn't have problems and B) it's supposed to be study leave now. DS3's school has had 'drop in' revision sessions over half term and DS has attended two on Tues and one on Friday on subjects he felt he could benefit from. For him that was English, history and music. There was a maths session that clashed with music but, similar to your DS, DS3 is confident about maths.
Next week there are 'drop in' revision sessions in school time and after school most days but none are compulsory.
His school did have a few compulsory after school sessions with selected underperforming students between Feb half term and Easter, and the super strict Chinese teacher had a weekly strongly advised session for the whole class. DS3 felt he got some benefit from those, though.
Time to just ignore these sessions mmzz I think. What are they going to do, give him a detention when on study leave? Or phone the HoY if your DS isn't comfortable with ignoring this teacher.
BTW, talking about sleep issues, my DS2 who has ASD, has always seemed to either need much less sleep than average or simply found it hard to drop off. He's always taken a couple of hours to drop off and woken with the lark, 5.30:6.00. Common with ASD but never too severe to need to consider melatonin. But at about age 16 this changed and he now goes to bed still quite late but can happily lie in until midday or later. I assume it's to do with teenage hormones kicking in. It was similar with DS1 but started much earlier, at about 13/14. And DS3 is still up with the lark at nearly 16. Up before me often even on school/work days. I'm waiting to see if that changes like it did with the other two. DS3 is still fairly short but has definitely hit puberty, but I think he got there later than most of his mates. I think the teenage 'slumber' has more to do with the timing of puberty than age, but DS3 is both young in his year and hit puberty fairly late so seems like an outlier.