Hey there, his length sounds normal and if he's gaining weight (a food scale is great for tracking this) then he should be good! But I did want to come and say that Mealworms are not really good for them. They have tough outer shells and don't carry all that much nutrition. Things like Phoenix Worms, Silkworms, and Dubia Roaches are all great staple insects, and you can treat with horn worms, crickets, super worms, and wax worms. His insects should also get dusted with calcium every other day while he's a baby. I use the TetraFauna ReptoCal with D3, which is readily available at any pet store. You don't really need a calcium with D3 in it though since it is thought they can't absorb the artificial D3 anyway.
It is very unlikely to find any of these insects in pet stores, so I order online. My favorite place is
http://www.rainbowmealworms.net because they have a very natural and organic focus, so their insects are fed well, which means your dragon will be fed well! The shipping is fast and they always give a discount if you go to checkout as a guest (fill in your e-mail) then leave the items in your cart for a few hours
Hope your little man keeps growing!