Hello again. He's been using tretinoin (actually, Treclin, which combines tretinoin and clinomycin, a topical antibiotic) for six months and it's the most effective treatment so far for the acne, which is pretty much gone apart from the odd spot. The PIH marks are fading, and he's good about using SPF.
You're right, there are other treatments recommended for PIH - vit C being one of them, also lactic acid. I've been cautious about him using more than the tret (apart from moisturiser and SPF) but provided you're not layering them i.e. tret at night, wash your face, vit C, SPF in the morning, perhaps it's fine.
Tret is available from your GP, as is azelaic acid. Both are listed on the NICE guidelines for NHS treatments for acne - google Nice guidelines, acne, and you'll find them. Azelaic acid is used as an alternative for those who can't use tret or adapalene e.g. pregnant. Even if you don't have active acne, if you ask on the grounds that you have scarring/PIH from previous acne, you'd probably get a prescription. It would be cheaper for you than using the online prescription service, I think?
I know the current situation is exerting enormous pressure on GP services, but don't assume that the NHS doesn't want to know about skin conditions. I wasted a lot of time and money trying to help my son with his acne with OTC products, before I came across the NHS guidelines and realised that there was medical help available even for quite mild (but persistent) conditions.
Good luck, OP, and just remember that time is your friend here - I've read it takes up to 18 months for all those melanin saturated cells to turn over and fade. But your efforts with skincare, especially the SPF will pay longterm dividends - no sunspots or wrinkles for you! And, in the meantime, you can use make-up tricks to camoflauge whatever you dislike.