Was he prescribing them to be taken immediately? If so, he was doing nothing more than treating a disease early based on a high probability of it's presence and the natural history.

Or was he providing a script, to be filled later if full "strep" symptoms developed? In this case I would use a term like "expectant" (though this often has connotations of a "wait and see" approach, with the expected - but not certain - outcome being death). It was only prophylactic in that it prevented another visit to the doctor.