pokemon? yu-gi-oh? digimon? any of the classics...however are you sure you want your 5 year old playing games? i know its none of my business but playing outside and stuff would be far better...just a suggestion...
hmm well, If you can put some homebrew applications on the DS, I would say Colors ( http://gbatemp.net/index.php?download=1217 )
If not, Nintendogs is a pretty and simple title.
I think he should play with lego and stuff that would enrich his own fantasy, and not let him depend on the fantasy of game creators... (maybe this in inappropriate)