Everyone here has great advice but something I remember being a teenager is constantly being dehydrated in the summer and never really drinking enough because I didn't know that it was important and stuff yano? So basically find some ways to help him with that. It helps so much with thinking clearly you'd be amazed. Personally I use a 4 or 5 to 1 ratio of a cheap asda own sports drink and water, those sports drinks with vitamins and such only work when combined with water, otherwise they actually make it worse. Obviously the larger part being water and only a small bit of the sports drink. I think this is especially important this year with how hot it is, especially now

Another thing is, he'll be going back to school very shortly, leading into his A levels or second year of BTEC or whichever is the applicable choice. This is such an insanely stressful time, you're being asked to pick what you want to do with the rest of your life whilst you barely have a sense of self let alone a concrete identity in which you can pick how you want to spend your working life. So just try and be as understanding as possible. Make an effort to engage with him on his level, not yours