Ohh, no soap, sweety.

Plain warm water will do just fine.

If I'm preparing myself or someone else for a long session, I find one of those syringe style enemas the best.
They are re-usable after cleaning, very easy to use, and also do not cost very much at all.

Most online adult stores use discreet billing systems, and also use discreet packaging, if you are hesitant about buying from a shop.

Orr..
Just don't worry about it.
There's a chance he'd like it.