Cambrils is served by Reus Airport. Ryanair don't have a flight to Manchester from there until 11 Sept. Their first flight to Birmingham isn't until 4 Sept. They've got a flight to East Midlands on Monday 2 Sep at €220.
Jet2's flight from Reus to Manchester is full (as is their flight to Birmingham). They've got a flight available on Sunday though (at 1150) but it costs €290. (There's a flight to Birmingham at 1415 but that costs €315. There's also a flight to East Midlands, at 1210, for €334).
Thomas Cook's flight to Manchester tomorrow is full. The earliest they're offering is Tuesday, for £257.99.
TUI's flight to Manchester tomorrow is full and there's no availability for Birmingham either.
With the school holidays just coming to an end, finding any flight at short notice is going to be almost impossible (and there's certainly no chance of getting a cheap one). If your son can find some cheap accommodation for a few days though, there are cheap flights available towards the end of next week.