You're one caring father. Good for you and good for your son.
The most effective treatment out there is finasteride, but it messes with your hormones so it's not for 17-year-olds. Besides if he wants to take that plunge you should let him make that decision.
I would suggest that he started off with revivogen for example. I don't know if it is easily available in the UK but it seems to be effective for early recession. Maybe combine it with minoxidil (rogaine). Minoxidil doesnt stop balding but it's a growth agent. And you will have to stick with it to keep your results. So it's a big commitment.
The easiest way is to use the guide to treatments on this website. It is easy to use and gives you a good overview of the treatments available.