I would just ask the teacher for a chat. If that fails talk to the senco. It's a bit lazy of the teacher not to adapt the h w to a child's ability. It is very demoralising for them if they can't do it.
Discuss it with his teacher; he may well be capable of doing it but will not work independently, particularly if he is receiving a great deal of one-to one support. This is a common problem of children of all abilities.
Not sure what the time scale should be, but could you say "we're going to spend 30 mins on the homework and then stop" ie put in a reasonable amount of effort (the time will depend on his age), but also have some rules?