1) If your son is under 13: It is ilegal if you create a account for a person below 13 years of age, you will have to pay fine if you do so. It is recomended if you create a account with your imformation and restrict mature content and set pass word for in-app purchases...
Check this article out:-
http://support.apple.com/kb/ht1904
2) If your son is above 13: you can create a account with his imformation. You can save your credit card imformation on his account.
Hope this helped you!
HDash_Tech