Can you speak with your GP or health nurse about it?
Where is he roughly on the charts do you know?

I think things like full cream milkm cheese & yoghurt are good along with plenty of fruit & veg.
I have found that even though both of my girls have always been petite that DD1 looked slimmer around 2-3yrs old as she was a hell of a lot more active and it worried me as she was so little to start with but she actually had a decent growth spurt and evened out her height aswell so the MCHN had no issues with her weight at all. IYKWIM.