I also think this is awesome and hilarious. I bet once the school finds out the teachers will also find it awesome and hilarious. They probably won't be able to actually tell him that and they will almost definitely have to shut it down but it'll make a fun staffroom chat.
He will eventually get caught and probably will get "in trouble" and made to stop but getting in trouble isn't the worst thing in the world. Especially if he's got the gift of the gab and gives it the whole "but sir I'm helping prevent teenage pregnancy and STIs" and "I am being a business man".
He sounds like the kind of lad who wouldn't be embarrassed to have to talk about condoms (good for him) so what trouble is he likely to get in beyond getting a talking to from a senior member of staff who is having to pretend to be angry but probably secretly is really impressed?
If he was selling vapes or drugs it would obviously be a different story. And honestly in my opinion selling condoms is better than selling chocolate or fizzy drinks.
As far as what parents would think about their secondary age children buying condoms, many will probably be unhappy but as others have mentioned kids can access them anyway. Kids who want to buy condoms to have sex will not be stopped from having sex by lack of a condom - at least not in the long term.
When your 14 year old goes to have sex with her boyfriend and they don't have a condom, hopefully they won't do it. But only because they can get a condom and do it tomorrow instead.
If you somehow manage to succeed in completely limiting their access to condoms then the teenagers who are "in love" and very hormonal are not going to say "oh never mind let's wait until we are 16 or even better 18 or maybe until we are married and settled down".
Their brains haven't developed yet. If your teenager decides they want sex then the best you can hope for is they're sensible enough to do it safely. If your teenager has the foresight to buy condoms then they care about their sexual health and are probably also the kind of person to care about consent and mutual pleasure etc.
If your teenager understands and cares enough to pay a pound for a condom then you've done something right.