dual nationality kids - which passport

I'm not sure this is in the correct place, but here's my question. My son (16) has both US and UK passports.  Currently, we're awaiting a replacement UK one. He has a school trip into Europe. (yes I know bad timing but best laid plans go awry). Does anyone know if he can travel on his USA passport? Will he have trouble getting back into the UK on it?
Many thanks,

If it was me if check which countries he's travelling to in Europe and see if they require visas for Americans before he travels.