I'd say that he needs more appropriate activity. Little boys have LOTS of energy and they need to burn it off in appropriate places - like running outside, climbing on playground equipment, etc. It sounds like your son needs even more of that than most. I would take him outside to a park right after breakfast and let him run his little heart out (bring balls that he can kick and chase). After he's had a lot of exercise, he should be more content to play somewhat more quietly in the house.
I would also check his diet - is he eating much sugar? That can also have a real impact on behavior. I'd start him out with a filling, healthy breakfast (slow cook oatmeal, low sugar cold cereal, eggs and toast) and make sure that his lunch is similarly low-sugar (beware of hidden sources like juice and sugared yogurt). Also, how does he sleep? Kids who aren't getting enough sleep often are hyper. I'd make sure he was on a predictable schedule with plenty of time for exercise and a routine naptime.
When you are expecting him to play inside, I would take a few minutes to read him a story, play with him or set him up with an activity (and realize that a 21 month old won't be able to do anything for more than about 10 minutes). If you periodically check in with him, he might be more content to play independently.
Good luck - if you try all of this and he is still bouncing off the walls, I would talk to your pediatrician.
Momma to a 21 month bundle of energy