I obviously don't know the circumstances surrounding your son so my points here are more general than specific but may be of some assistance.
Schools obviously need to consider the rights of all students and sometimes the behaviours of some are detrimental to the progress of others.
Sometimes these behaviours are caused by difficulties that the student has, other times they are caused by the student just being a naughty individual.
Whichever of these it is the impact on other students can be the same.
Where it is just a very naughty student then the school can obviously go through the sanctions etc. ultimately leading to permanent exclusion of necessary.
Where it is of no fault of the student, medical or whatever, it's obviously much much more complex - the school need to ensure that the student is supported in every possible way to be in lessons and learning but, also, allowing other students to learn.
If it isn't possible then I have seen schools move to an adjusted timetable - such as mornings only or afternoons only with a focus on particular subjects in that time.
When I've seen this it has always been part of a plan agreed fully with parents - either, as Hup said, an interim programme to try and get things back on track or a permanent programme to get through to the end of school if the student is near the end of Year 11.
Neither of these approaches seem to match the OP's post which concerns me - in such a complex case I would expect the parents to be an integral part of the process.