My 21 month old hit his head about 2 months ago a knot is still there, should I be worried?

Probably not. This is likely a slowly resolving hematoma, which is a blood collection under the skin, basically a significant bruise. Generally in children this old there will not be health consequences, but if the lump calcifies and hardens, it can be present for months or longer. Typically a cosmetic issue more than anything.
No, don't worry. A lot of times bruises get calcified (calcium deposits) and it forms a knot. It usually takes 6-9 months to resolve completely.
Head injury. No. If he has not had any symptoms for 2 months, the lump is probably just a resolving hematoma or bruise which will be reabsorbed slowly.