No idea about Wilmslow, I live in Stockport, ,my son gets the must at 7.05 ish - public bus to Alty - takes about an hour. He prefers it to the school bus from East Didsbury.
In fact- if you could get him to one of school buses ( they are actually for the girls but boys get on them too) that might be better. I'd suggest on route (Heald Green/Airport/Cheadle if you could get him there?) rather than him getting to ED for 7.15 then sitting on a bus to school for an hour.
Check on TFGM website school services for the school routes under AGGS. then we used it it was fairly reliable though the tram work at the airport means the bus leaves earlier than it used to.
The school bus drops on Hale Road right near school- but if te option you choose drops off in the centre then there are hordes of boys and girls all going the same way, he woiuld be perfectly safe. My son does that walk here and back each day- its a great social time ( and I suspect 'meeting up with the Alty girls time'
Let me know if you have any other questions- it was my major concern when we accepted the place- but it all works out somehow!!