A friend of mine had one I think I might copy (with the appropriate variant depending on whether we find out ours is a boy or a girl--another two weeks or so).
She already had three girls, so she had them line up on the sofa, each holding a balloon. The oldest girl had a pink balloon that said "It's." The next had a pink balloon that said "a." And the littlest had a blue balloon that said "Boy!" I'm not sure I trust my youngest with a balloon right now, so I might do the same with the three oldest holding balloons, and have the youngest hold... something else. Or I could give her a balloon with just an exclamation point and see how she does with it.
I think any baby item that's in blue or is obviously boy-ish would make a good reveal. Baby shoes, baby toy, baby clothes, whatever. Could be held by somebody or just by itself. But if your 2-year-old is willing, doing something where he's holding the item, or holding a sign, would be really cute and probably a great picture to share with the new baby when he's old enough.