I really cannot offer any other suggestion than what you are already doing, namely, playing with the toys in front of him and getting him a large variety. But, I tell you, none of my birds really play with toys. They all have some and, when I first put them in (and if I am lucky to have chosen one that they like) they will 'play' with it but, after a few days, they forget all about it and hardly touch it from then on.
The thing is that parrots don't really play in the wild, they chew, they shred, they forage, they bathe, they interact with one another, etc so playing is not what one would call a natural behavior in them. Having said that, what I do is look for toys that they would like to chew or shred so my cockatoos get wooden ones while my amazons get cardboard boxes and they all get what I call 'shreddies' or 'chewies' which are toys made out of material that is fun for them to destroy (they particularly like the colorful ones made out of many different pieces in natural materials - like these, for example:
http://www.amazon.com/Planet-Pleasures- ... toys&psc=1http://www.amazon.com/Super-Bird-Creati ... arrot+toyshttp://www.amazon.com/Planet-Pleasures- ... toys&psc=1You might also want to try kabobs because the little ones love to chew on them:
http://www.amazon.com/Wesco-Mucho-Grand ... arrot+toys