It featured Tony as a pre-schooler and Good Dad Howard. No one thinks anything of the bumps and bruises Tony seems to get, believing it's just part of growing up. It's not until Tony comes walking in on a broken leg (from trying to fly, maybe?) that Howard takes him to the hospital, where Tony proceeds to show everyone in the ER how he can "wiggle his leg all over". It turns out that Tony has CIPA (a rare genetic disorder that makes him unable to feel pain.) Once they are back home, Howard has a long talk with Tony about how important science is and get his interest focused on lab work rather than being more athletic.
FOUND! Link below in comments.