I Have A 10.5 Yr Old Son Who Walks Like

I have a 10.5 yr old son who walks like he is 90 yrs old he walks very stiff like, feet hurt , im tired he over 5 ft tall and weights in around 140, big I know.. he doesn't eat like you would think but he does kinda fast when its time to eat, he drinks tons of water he prefers water to drink over anything else. where/what would you recommend starting with this boy of mine?