My father is having an adverse reaction to allopurinol. Can he stop taking it before the prescription is finished? He has a fever of 103, is sweating, he's very weak and he's refusing to go to the er.

He. He definitely needs to stop the medication. Inform the prescribing doctor of the reaction. He hopefully is better at this point but if not he needs to see his doctor or go to the er before things get worse.
Yes, . Yes, whenever you are concerned you are having a reaction to a medicine, stop taking it immediately and call your doctor. However, with his symptoms of fever he should be evaluated immediately, so please urge him to be seen.