I don't doubt that age at all. I've personally seen dragons at a local pet store who were 5- 6 months old and only 7-8 inches. If they are not fed right, they won't grow.
He is very, very small for his age. While he will grow with proper care, its unlikely he will attain a normal size. Make sure he is eating all the insects he wants everyday, he needs protein to grown.
Can you go over care with us? These questions will help us help him.
How old is your dragon?
How long have you had your dragon?
How long is your dragon?
What is the sex of your dragon?
What size enclosure do you have your dragon in?
What type substrate do you have on the bottom of your tank?
Do you use UVB lights?
If so, Is it a coil, compact, fluorescent tube, or Mercury Vapor bulb?
What is the brand name and number of your bulb? Wattage (if MVB)?
How old is your
UVB bulb?
How close can your dragon get to the UVB?
Do you use a separate basking bulb? What kind and what is the wattage? Is it a white or colored bulb?
What are the basking temps?
What is the cool side temp?
Do you take the temps with a stick on thermometer, a digital thermometer with a wire and a probe end or a temp gun?
Where exactly are you taking your basking temps?
Do you use a heat rock or heat pad?
What do you feed your dragon? Please be specific.
How often do you feed and what time do you feed (morning, afternoon, night)?
Do you gutload (feed) your crickets, worms, etc?
Do you use vitamin or calcium supplements? What brand(s)? How many days a week do you use each of them?
Is your dragon having regular bowel movements (poops)?
Do you
bath your dragon? How often?
Do you mist your dragon or offer water other than in the
bath?
Does your dragon share an enclosure with another dragon?
Have you gotten a vet check and fecal done?