Oh, the list is endless! Here're a few ideas.
A wind indicator is useful. I have mine mounted to the front crossbar on my Hobie Getaway. A H14 or H17 with no jib could mount it on the mast.
https://www.amazon.com/gp/product/B000V ... UTF8&psc=1A soft cooler bag is nice. I have a Hobie Brand, but I'm sure it's expensive.
https://www.amazon.com/Lifewit-Collapsi ... NrPXRydWU=A waterproof bluetooth speaker is good, if he likes a sound track. This one floats, has good battery life, is small, and isn't very loud. I just want to hear my music, not broadcast it over the lake.
https://www.amazon.com/gp/product/B00RK ... UTF8&psc=1