My son had a fever a few days ago and now has sores on his throat. can it be from sinus drainage if the fever is gone? or is it the dreaded strep?

Strep is possible: While strep is possible this could also be a virus. Specifically either hand, foot and mouth disease or gingivostomatitis. Both are caused by viruses. I would suggest you have him examined to better clarify what is going on.

Ulcers = sores? : Strep doesn't cause ulcers or sores. It causes difficulty swallowing, fever, redness, swelling and pus in the throat. If vesicles or ulcers or sores are associated with the sore throat, the infection is invariably viral.

Coxsackie virus: Small ulcers on the back of the throat (retropharynx) suggest the possibility of infection with coxsackie virus (herpangina), but without seeing these it is impossible to be sure. It may also be other conditions, but not likely to be strep. The best way to be sure is to visit the doctor and get tested.