As a fellow short-a*** :) my usual response to any height related issues is to wear ridiculous heels - although whether that’s a good idea in your shower I’ll leave to your own risk assessment haha. But position wise, if you get him to kneel but then put his hands behind him and sort of lean on them with a back bend so his head is thrown backwards , if you see what I mean, that will bring most of his body within firing range and also allow him to keep his face out of the way if he’s not ready for that yet.

Or make him lie / curl up as best he can be scrumpled up on the floor of the shower, but cuff his hands upwards (like to the taps or the temperature control whatever you have - they’re usually at about the right height halfway up the wall). That will keep him low down beneath you but also raised up enough and sort of lifted and better able to fit and balance - and bonus points for restraints. Then you can stand over him so he’ll have to look up at you while you do it (a positive I would expect) and you’ll probably be able to be above his head / face and aim there too if that’s what’s wanted.

If you strap him up and then take your time you can build the anticipation beforehand as has been said by others, and then afterwards you can easily hose him down while still restrained if that would be fun too.