Luke loves Legos and Bionicles. And he really likes K'nex as well.
But he ~does~ play with them. We've got parts scattered all over the house and he wonders why we can't put them back together.
You could get him the Lego Star Wars game - do you have a gaming system? It's a fun game. Easy enough for even me to play. You get to blow up lots of lego stuff.