If Finlay went home during the actual school day, the school would definitely be breaching their duty of care.
I would guess (but you need to check), that by the school offering further after school activities, that they have in effect, extended their duty of care, until the said activity comes to an end. Personally, is still expect to see my DS there, definitely, until the end of said activity.
I know I allowed my DC were allowed to walk home sometimes in year 6, but more often than not, I'd be there to pick them up. Between us, we worked out the safest way to walk home and they both carried keys.
However, with a child with special needs, only you and the school can honestly say whether he should have been safe to let him go home alone.
You need to make it categorically clear to the school what your expectations are. I'd also contact social services and find out how you stand, legally, with your son. They have been very helpful to me in the past.