One of my sisters was like that...really, really active from day one (she walked by nine months and was running full tilt by the time she was eleven months old) and always super thin, to the point where the doctor put her in the "failure to thrive" category when she was a year old - she only weighed fifteen pounds. Didn't exactly make Mom feel good...
We used to give her whole fat chocolate milk instead of just regular milk and plenty of snacks. Maybe the sugar wasn't the greatest, but it helped!
She was perfectly normal by a coupla years later. I think she was just reallllly busy and burned everything off!
I really needed to hear that, sis...I'm really anxious about it, because he just looks so skinny. But like I say, it hasn't slowed him down at all! Heh.
I make his carnation instant breakfast with whole milk. And the malt will be made with heavy cream! And after I cried last night, he's been trying extra hard to eat everything I give him! :-)