Your DS might be learning more than you realize, too. Both of my kids come up with seemingly out-of-the-blue ideas and explanations, synthesized from bits and pieces they saw in a museum, or read about, or heard somewhere. There's a flash of brilliance, then there's a flashback to toddlerhood a few minutes later.
Some of our friends don't believe DS is gifted, because he doesn't fit the "gifted mold" that they expect (intense about a subject, serious, high achieving, etc).
IMO, playing with Legos is a work/learning experience for kids. I'd suggest more free, open-ended down time. Let him get bored. Let him play with Legos or scribble all afternoon.
(And if you do want to start a passion in your son, consider introducing him to Minecraft. On the other hand, be careful what you ask for. And maybe do a little reading about it first.
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