You should try him with different types of toys - things that make noise (rattles are Kiku's favorite), things that spin, things that have moving parts, rawhide for chewing maybe. My general rule of thumb is that if I would find it interesting at least for a short time, she will too. This means I avoid buying those premade toys that are just wooden balls on wire, or otherwise not-very-interesting premade toys. Plus it's fun to make them yourself
Some toys that I've made for Kiku:
Get those little wiffle balls (I bought a six pack from the dollar store) and find some beads that you can force inside (make sure they can't come out). Then hang them in the cage. Kiku likes to shake these and spends time trying to get the beads out (of course, she can't). These are also good to stuff with other things (treats or shredded paper) and you can thread beaded leather through them. Can also be used as foot toys.
Uncooked pasta noodles in different shapes and sizes strung on leather or some other safe material are fun for chewing.
I got a half cocounut shell and hung it in her cage and filled it with interesting acrylic toys and beads (all safe). She likes to forage and throw them around.