Hi Mabs: not sure of the condition / circumstances which surrounds your appointment with the physio, but in case it helps,this is the experience I have. My ds, now just 2, wears an AFO (ankle/foot orthotic) which means splint in any other language. He has one for daytime and another, less rigid, for night. They have been very carefully made and fitted for comfort and he has never objected (and he's a pretty feisty character), tho' he is younger than your ds, and has worn one since 14 months. We have always managed to find high street shoes that fit on top, generally ankle height trainer types. Last winter he wore Clarke's ankle boots with a velcro fastening across the front, sandals have been harder because we have found that the ankle height is much better for fastening securely. You'll need to go from the orthotists to the shoeshop to get shoes that fit on top, so don't go splashing out in the meantime! He wears 100% cotton socks underneath.
He wears the splint all the time he's out and about, unless we take his shoes off for a bouncy castle etc, it isn't at all complicated, just fastens with velcro straps, and his childminder has no difficulties with it.
You (he!) might get a choice of funky designs on the plastic, so far we have had teddies, racing cars and dolphins! Or you can choose a plain colour.
When you go to get them fitted, ask the orthotist to put non-slip soles on them: the plastic is quite slippery and if ds takes of his shoes, or gets out of bed and walks around in his splints, it helps make sure they don't skid.
Don't know if it's the same for your ds, but the splints have helped make a big difference to my ds's feet, holding the soft tissue in the right position as the foot grows. (He has a missing fibula, talipes - or clubfoot - type foot position and part of his foot missing, and had surgery last year to enable him to put his foot to the floor).
Won't be on the board much over the next few days, but if you've got more q's that you think I might be able to help with, just say!
Good luck