From reading your updates OP, your husband has form for this type of behaviour and admitted to using prostitutes prior to meeting you. Given your updates, I'd say that your husband regularly uses prostitutes whilst away. Knowing what you do, now finding out he's likely been to a Thai massage parlour, it shouldn't have come as a surprise to you.
Of course your husband will deny any wrong doing. He's wormed his way out of situations before, and will definitely try to do the same again. I definitely wouldn't be confronting him whilst away but I'd be asking him matter-of-fact and in calm manner how he found the massage. You're not accusing him of anything, you're simply asking him how his massage was! If he becomes defensive or denies having been for one, then in my opinion, he was given more than a massage!!!
You need to think about what you want going forward. Deep down you must realise your husband uses prostitutes, he did before meeting you and he's continued periodically throughout your relationship. I don't see him stopping either, I'm sorry to say.
Regardless what you decide to do, you need to protect your sexual health, which means getting a full STI test. You'd hope your husband has used condoms, but you have no way of truly knowing.