To the OP, and the question posed: You're the client, and can take a moment for any sort of comfort or hygiene adjustment that makes you feel better - just remember, the clock keeps ticking.
Your post's wording is unclear about whether you have never eaten ass at all, not yet eaten his in prior app'ts, or if you're well experienced at it, but just never used it as the first kiss. 💋 It's a bold move, and if you've hired the right man, and you have talent for it, it can lead to a memorable session.
I hope you did research on the masseur to confirm he's also a provider of full escort service, that rimming is in his into's and that you intend to pay the rate for that offering.
Toss My Salad 😂