Fevers can be caused by a number of things including infections and allergic reactions...I personally wouldn't administer anything without talking to a vet first. Giving drugs like bute can help, but can also mask the symtoms making diagnosis more difficult.
I wouldnt feel comfortable giving my horse anything without at least talking to the vet about dosage. Bring this up to your vet the next time he comes out. Maybe he can tell you what to do without calling him