From a PP (not trying to nitpick, but typing this post from a point of view where I thought these things would be easier than they have been, 2-3 months on from the health issue that affected my DC's performance).
Im not someone who believes that children should have 100% attendance but these things are only solved when the school and parents work together. The school can put steps in place to make the transition easier.
They can, but you would think that in the case of concussion and injuries that are still under a hospital clinic, this should be the bare minimum that needs thinking through, and instead the HOY has said nothing and the receptionist refuses to log injury absence when they've decided 'unauthorised'. So it doesn't look as if they will pay great attention to constructing a managed plan to support the student without real care, that the parent and medics need to ensure is put in very soon.
He doesn’t need to wear uniform, he can have a reduced timetable to avoid busy bus routes, he can take lessons in a smaller room or leave lessons a few mins early to avoid the corridor, have a ‘buddy’ to carry his bag.
The GP advice and extracts from my DD's medical records (from a while ago) showed some clear needs that could be implemented. I requested this of school on email initially. It's taken 3.5 weeks of January and a lot of chasing to actually have these points passed on reliably for action in classes. One of the recommended adjustments has also not been granted, although the rest are, so in the spirit of compromise and keeping communications open, I am currently seeing how it will go. So don't assume that all the above will be waived through even if they look as if they couldn't be more sensible for your DC and their physical condition, to you.
But it sound more like OP and school are at loggerheads and instead of working together to find a way to get him there safely and confidently.
There are 2 sides to every story, but the school seem to be the side that began to be obstructive and throwing their weight around.
OP, bear in mind that this may not be the last of the absences that you will see this year (possibly connected to this accident) - they sound already to have decided you are exceeding a norm, even though proper healing and being on board with a well managed return to school is something worth standing your ground on, because it may have worse consequences if rushed and done under duress. When nothing was happening between mid Dec and mid Jan for us, this really did ramp up anxiety and paranoia in my DC more. You can manage some levels of it at home, but if they feel nothing is happening despite a lot of effort to pass on needs as they change, then that fear was at levels I was out of my depth with, and it could have been contained with more help.
To be able to have all of this documented - definitely physical fitness and possible effect on work for a while - in case of needing help from others (GP, clinic, dental people...), you need to explore a health plan and, at least, adjustments in school to start immediately, not in 2/3 weeks, to help get back to a workable place.