I have a seven (maybe even eight) month old girl, Cream that is the size of my three month old girl, Eevee (who is pretty petite). Cream had baby fur when I got her, but barely grew. She looks like she's half the size of Silver, who is only a month or so older than her. However, Cream is definitely way too big to be a dwarf (I truly can't imagine a rat being that small... wow) so I guess some rats are just different sizes. Cream's sister, Peaches would be the same size, but Peaches is fat. Peaches is still shorter bodied, though... just wider.
In addition, to show how the sizes vary in my house, I have a three month old boy, Cricket that is bigger in every way than my two six month old boys, Gus Gus and Bartok. Then I have Meeko who is the same age as Cricket and less than half his size!
Your boys smallness could be one thing or a combination of things. For example, he had a rough start in life and probably didn't receive as much nutrition as he needed which would stunt his growth a bit. He could also just be naturally small, like Meeko and Cream. Or he could just be hitting a slight growing roadblock. I think Gus Gus and Bartok stopped growing for a while around that age and then abruptly doubled in size almost over night.
I personally wouldn't worry too much about his size as long as he looks happy and healthy.

He looks so much better than he did when you got him!