Any tourist attractions near you, or fruit picking farms?
How about kids' play schemes and holiday clubs - he might not be old enough but I'm fairly sure you can get a basic DBS check at age 16. He could email the ones locally and find out (sports centres for example or specifically holiday clubs, or after school club providers who often run a holiday scheme, if he googles he'll find names of them and can email his cv to head office).
There is one play scheme in my area that only runs in the summer and it is entirely staffed by young people under 20, it is wonderful as they all have a fantastic time a bit like an American summer camp but not residential, ssentially it's just playing with the kids - water games and archery and football and stuff - and stopping any trouble amongst the kids. Lots of lads working there, not just girls in the summer).
Or maybe he might get a few shifts as a lifeguard at your local sports centre? You used to be able to do this age 16, not sure if rules have since changed, although he may need to do some training.
Or, could he advertise on local Facebook and offer to wash cars, walk dogs, babysit, mow or water lawns while people are on holiday (they would need to leave lawnmower in a shed he could access, but neighbours might be willing if you know them). He could leaflet in your local streets. If he is cheap enough, it might be turn out to be quite popular.
Last thought is whether there are any local gardener or decorating businesses that would take him on for the summer. He could offer himself fairly cheap, learn the trade a bit, and get some skills into the bargain.