Hello,
Thanks for writing to Health Care magic, I am Dr , Muhammad Ahmad , I have read your question closely, I understand your concern and will be helping you with your health related problem.
In such an accident he should  not be putting any weight on the knee , when there is a direct impact on the front or side of the knee  and there is swelling , it is always advised to visit a doctor who might want to get a detailed checkup c.ray or even a  
CT scan of the effected knee because in such a hit, breaking of cruciate ligaments of the knee is  very common and needs proper specialist care and treatment to get cured.
Till you see your doctor you should wrap crepe bandage wound his knee, ice it every 3-4 hours, ask your son to rest as much as he can, and 
tylenol 250 mg can be taken in pain
I hope this answered your question, If you have more queries I am happy to answer you.
Regards.
Dr.Muhammad Ahmad