£200 isn't enough I'm afraid to buy and run a PCP.
Secondhand you might
be able to pick up an S200 (an excellent rifle for HFT) for a little more than £200. Then you'd have to buy a scope. Say £60 for a reasonable starter scope. Then you'd need a source of compressed air; you'd possibly be able to pick up a 3 litre 300 BAR bottle with adaptor for about £130 (I'd recommend not bothering with a stirrup pump, they're not significantly cheaper than a bottle and they need a bit of weight and strength to get them working) . Add a gunbag and pellets and you're talking over £400 separately. If bought as a job lot secondhand you might
get the kit for £350 or so.
So, two options then. Save some more money or buy a quality secondhand springer.
Sorry to micturate over your son's cornflakes.
I think it's brilliant that he's saved so much but unless he gets very lucky those are the unavoidable facts.
Oh and CO2 isn't really an option. I do know some people have had some success with some very careful tuning of CO2 rifles but without that they're way too temperature variable to be of any use beyond casual plinking and close up hunting. It'd be a waste of his hard earned/saved money.
The Brocock Concept Narky Mark is selling may be a path into this but you'll still need a bottle.