Help with my bearded dragon?

I have a two year old beardie. He is quite small only 30 centimetres. Is that normal? He usually doesn't move around, he just stays under a log in the cold part of the cage all day and night. Every Morning i have to put him under his heat lamp to warm him up and he never uses his heat mat. He also doesn't eat crickets, only locusts and eats a lot of veg. He hasn't gone toilet in two Weeks and im getting worried. please help