If this is something you really want to do to please him but are experiencing a psychological block, you may want to try this suggestion to start off. It might release your inhibitions.

Drink a few glasses of water and wait until you have to pee so badly that you feel as though you'll burst. If you have a shower/tub combo, have him lie down in the tub, straddle him and turn on the shower as casually as if you were going to take one. Trust me, you won't be able to hold the pee.

After a few times of doing this, you might feel more comfortable with the out of shower shower.