How can i get my 20 month old son to stop biting on the inside of his cheeks he does this constantly and he bleeds?

Chewing on cheeks: Make sure he doesn't have a canker sore or cut in his mouth. If so , use orojel on sore. Otherwise try to distract you child from chewing on his cheek by giving him a banana or apple slices to chew on. Suckers might also help.

Habits: Generally intentional chewing on lips/cheeks are a nervous habits. If left alone (don't scold, make an issue etc) many times the habit will go away on their own. Otherwise seeing a dentist to check for mal alignment issues would be recommended.

Biting cheeks: This is a tough one. I'd start by making sure he is not iron deficient/anemic by checking a hgb, serum iron and ferritin. Almost could be a pica type illness where he feels need to eat non-nutritive substances. Also see dentist to make sure no malocclusion of teeth leading to grinding and biting. He may also be able to suggest a mouth guard of some type to make the biting difficult.