I would get him an M1 Abrams. Arugably the best tank in the world today.
matomart sells them for about 180.
if you want to upgrade there is another company selling them for a few hundred more with metal suspenstion and wheels and tracks. I can ask my friend where he got it if you are interested.

the M1 is such a recognizable dominant tank. And its big and looks cool. Would send that little kid running for cover.