Ritalin is a stimulant medication that can cause a number of unpleasant side effects. Sometimes, especially when the medication starts to wear off, some children experience a "slowing down" effect (although some children experience the exact opposite and become more hyper), so it really depends on how your son responds to the medication.
You might find this information helpful, as it discusses the possible side effects of Ritalin:
I can't provide you specific medication information but I would advise you to consult your son's doctor to discuss his symptoms and to see if there is a different medication that he can take that will not affect him in this way. There are alternatives (you can read about some of those here:
If he is not completing his school or homework on time, then you might want to discuss this with his teacher to see if he requires extra help, or if there are steps you can take at home to help him complete his work on time.
Best of luck.