1x1 training will train different secondaries for guards and forwards, in particular it will boost IS if you do 1x1 for forwards.
It doesn't matter if he plays SF or PF, as long as the sum of his minutes as SF and PF is 48 or higher. For instance, if he plays 26 mins in one game as SF, and 32 in the next one as PF, he will get full training if you do 1x1 for forwards.