Given that the school are aware of difficulties and EHCP is in progress it is also unreasonable that they keep him in at break.
If he can complete work then they should be providing the same support out of break time. If not, it's a punishment for known SEND.
Also if break is downtime (some DC want to be kept in to avoid social communication even if expected to work) it is counterproductive. Even if this is the case he needs support in break times, access to quiet areas etc not being kept in to complete homework.
This is unrelenting pressure and I would be worried about burn out. Mental health is for life. I speak as someone whose DS1 got EHCP and specialist school (following tribunal) too late and has been a full time carer for over a decade now and will be til I die.
I did things differently with DS2. Because he had tried and failed secondary and most importantly had an EHCP, the LA funded internet school as alternative education. I had to be his unpaid LSA but they also paid for tutors for subjects like English and history where he struggled. He excelled at maths and physics. No telling off for any subject.
His mental health was not damaged and his confidence grew even though he and his behaviour was unchanged. ASD and ADHD. He had some social skills training at primary but it didn't take. I still have to listen to monologues.
He went to 6th form to study further maths, maths and physics and is now in his first year of a masters in mechanical engineering. Disability support is provided as a matter of course. Maybe he'll get a job, maybe he'll crash and burn after uni but I did my best. He wouldn't have the same opportunity, even if it turns out he can't take it, if I had not intervened.