But a return to being serious and attempts at being helpful: if you must do anything, warm water (and nothing else) will suffice. Do NOT add soap or anything artificial, I wouldn't trust it. Odds are too great of a poor reaction on a mucous membrane so delicate. Getting the temperature just-right is going to be the hardest part. Aim a few degrees above body temp, testing on your forearm not your fingers, or use a thermometer rated above 110F since that might be around where you're shooting. And keep in mind that whatever container you transfer it to will steal that heat gain away soon as you pour it in, so strongly try for applying it to him
with the same container it was heated in. E.g. a coffee mug. For application I have no idea, since I've never felt the need to do anything remotely like this -- the carnality of scent and taste
is the appeal for me, so I would never wash away what he makes there. Nay, I relish it as a treat. But if you simply must suck an odorless, flavorless (joyless?) dog dick, well, then you can try carefully pulling his sheath back and dipping in a mug to give it a few gentle swirls, a spray bottle set to mist not stream, a large-capacity sharpless syringe (30mL+ luer-lock type) or other assorted improvised veterinary irrigation tool. I still kinda feel like this is all folly but its a start.
Mind you I'm not trying to knock anyone here. Not saying you're wrong, OP. But me? I still think y'all are defeating the purpose by all this prep. And my money's on he ain't gonna like your efforts. Ultra-hygienic = boner killer in dogs. In time you'll find its a [happily] acquired taste, so better to just give it a chance? Practice makes perfect. I've been doing this enough years I don't even taste the metal anymore, just the delicious salty salinity. But my guys are on raw diets too, and that makes a world of difference.
Try it but plan on having to suck it up (pun very intended) and enjoy him raw & natural. Who knows. Maybe In a few weeks come back to us with more a page from Dr. Strangelove - or, "How I learned to stop worrying and love his smegma."