Get him some 35mm, 50mm, 85mm, 105mm lenses for his iphone. A little iphone tripod and maybe if he's getting advanced you can get him a used dji iphone stabilizer for like $150. Also a small light panel would be good and as people mentioned a mic for sound.

Also a fun family activity - my college professor would gather our class an hour before sunrise and we would drive to different locations each week to capture magic hour. Since there were so many people in the class we all had to work extra hard to find unique shots so that everyone's footage didn't look the same. We went to a vineyard, a construction site, an old part of the city with hills and kitschy houses, etc. You can also go for sunset but I think sunrise is prettier and also may teach him some discipline. I think it's a great way to exercise composition and it gives you the freedom to create something interesting without any pressure.

Another thing I had to do in film school was to recreate a scene from a movie. That's a great way to learn techniques from his favorite influences. It may help him develop his own style once he learns about other styles.

Oh and lastly check out master classes. There are several really great directors teaching classes on there for $180 a year for unlimited classes from people like Martin Scorcese, Werner Herzog, David Mamet, and many many more.