What was wrong with my son?he seems small for his age.does having an adhd connects with it?

ADHD : Physical size is driven a lot by genetics, the physical size of both parents, also nutrition. Genetics also predict growth timing and growth spurts if your son is on adhd stimulat medications, that can inhibit his sense of hunger and growth; so some prescribing doctors recommend medication vacations to help with weight gain and growth. Talk with your pediatrician about your concerns.

Talk to Pediatrician: He could be short-statured; some adhd meds can reduce overall height--but usually not by a lot; or there may be another issue present. Cannot delay evaluation too long, as once growth plates close, cannot do much. See pediatrician soon.