If its just for riding to school or a passing phase then I’d say halfords, they do everything from very basic up to some fairly good voodoo bikes which are getting into the entry level “good” bmx’s
If you’re feeling flush or he’s planning to use it in anger then you’re looking for as light and strong as possible so full chromo frame is desirable, with chromo forks and bars pushing the price even higher. At 11 years old bike size may restrict what tricks etc he can do. My boys first half decent bmx was a gt fly which was nice and compact but he struggled to do much more than short manuals and bunny hops on it. He got a new bike for his birthday which was full chromo and a longer top tube and his abilities have dramatically increased in the few months since he got it. Last week me was mastering foot jams, this week he’s on to bar spins and I’m slightly jealous!
I’ve kept his old bike to wobble around pump tracks on 🙂
Ps whitstanley bmx or chain reaction have decent ranges of decent bikes