They say old habits die hard. Gulping down milk is real easy whereas chewing food is time consuming n boring maybe.. You know that only milk is not going to fulfill his diet requirements, but he doesn't. U have to be persistent and not cave in. Put your foot down, limit his milk intake.
Try offering food in fun ways like a tortilla, burger, Frankie with veggies inside. Make it interesting. If u keep at it long enough I'm sure he'll catch up
His weight and height is fine for the age, so dont worry