Have you thought about just testing? If either of you have had sexual activity with others, testing is a good idea.
Performing oral on a penis puts you at risk for oral gonorrhea and syphilis. A syphilis sore has to be present in order for it to transmit, so if he didn't have sores, you don't have to worry about that one.
Gonorrhea can be present without symptoms. It's less likely to transmit orally than from vaginal or anal sex, but it's definitely possible. It's less common than chlamydia (which doesn't transmit orally), but it's still fairly common.
It's possible that he could get genital herpes type 1 from getting oral sex, but if he did, that's unlikely to transmit (it doesn't recur frequently, and rarely sheds). If you have oral hsv1 (about half the adult population does, and 90% never get any symptoms - think cold sores), you can't get it again in another location.
Having 2 partners doesn't put him at big risk, but if you are concerned, just ask him to test. If you've had any previous partners, you should test together, and then you can put the issue behind you. Lots of couples test together at the start of relationships. It's a good way to start, opens communication, and builds trust.