What could cause the really bad foot pain in the mornings and during the day after sitting?

Apophysitis: He may have apophysitis. See a podiatrist.

10 year old: Start off with a physician exam. Many 10 year olds will have heel pain as you describe. Look for inflamation of the growth plate of the heel as a possible reason. This is treated with exercise, ice and rest and can be hard to resolve quickly...See you doc and ask.

Is it in his heel?: People usually describe pain in 1of 2 places as "heel pain". Pain on the bottom is typically due to plantar fasciitis (www.Drmarkgalland.Com 1/16/13), while pain on the back may be sever's disease (www.Drmarkgalland.Com 9/12/12). Either of these could be the case w/your son. If it's not in his heel please ask again w/ more specific info on the location. Hope this helps and thanks for asking!