I'm a live and let live kind of lover...but my concerns with this are practicality mainly...
As Gaggia said...these days every woman knows that douching is a no no. So taking into account he's already said he'll be pre prepped with a full bladder, not only will you be douching with urine, but more than likely a forceful voluminous stream of hot piss hitting you internally like a race horse. And then gushing out everywhere uncontrollably.
And then you'll have to douche again with water and probably soap to remove any traces (I love my DP and I'm not a hater of men, but I'm sorry male pee is not beautifully fragrant).
Then as others have said, the possibility of even after that..it leaking out the next day.
I can see a host of health and cleanliness problems if this is a regular thing.
But then I am one of those unlucky lovers...cystitis if I don't wee after a rampant bonk, thrush during sweaty holiday sex, sand burns on the beach, UTIs from a bit of rough strumming, Ingrowing hairs from a Brazilian wax etc
The other thing is possible coercion, that people are picking up from his conversation of having practiced this many times...he is normalising something he wants to do to you, knowing you aren't yet comfortable with it. Only you know the situation op, but from the outside I think readers are wary of this.
And it should be a two way relationship so is he satisfying your kinks and alternative desires too? And if you say "it's a no for now" how will he react?
Or if you suggested something equally "off piste" like, "ok but can I take a big dump on your chest" how does he react?