Lemme step back to the question.

Should you use a condom from now on for oral sex? YES.

First and foremost, you might not catch something he had. STDs don't always transfer, so you may be lucky once. Secondly, a flavored condom will probably be nicer for you, and it may give him some extra friction he'd like. Finally? Safe sex is always the correct answer. There's a point you can 'know' if your honey is monogamous, but it's usually years down the line.

It doesn't matter if he never cheats again. It matters that he cheated once and you could be in danger of an STD. So use a condom, be smart, and be safe.