Not sure what your budget is and what accesoires your kid already has.
+A set of straplocks is always great. Schaller, Dunlop, Loxx.. can't really go wrong with the name brands.
+Behringer v-tone Bdi 21 pedal
Always good to have a direct line to the mixing desk with a bit of your favourite sound.
+A set of his favourite strings.
+A nice guitar strap has been mentioned, I like Marusczyk padded straps.
+D'Addario Guitar Maintenance Kit for Bass. Has every tool for setups to string changes.
+Mono cases bass gig bag. The best way to protect a bass on the go that's also portable.
Headphones for silent practice at home, a comfy padded stool, Focusrite Scarlett 2i2 audio interface for the computer, LESSONS or a workshop, a good guitar stand, Driver's license...
So many things! I tried listing in ascending order of cost a bit.
Oh last but not least, all bass players really really want one thing above all else...
Another bass guitar.