Oral sex is common and popular. It is not a big deal to have done it before. It is also unlikely he is walking around with a sexually transmitted disease from it- but if you are worried, why don't the two of you go to a public health clinic and get tested for them. Then you will know all you need to know-
It is rare that anything is contracted by kissing-- certainly you are more likely to catch a cold that way than most sexually transmitted infections. The exception to this is herpes- herpes can be transferred from mouths to genitals, genitals to mouth and mouth to mouth. It can also be transferred without being symptomatic ( showing a sore) although it is most likely when an active sore is present.
I don't know if you have gotten past this or not.. But whatever your present situation- I do think you need to forget about people's oral sex history and concentrate on just knowing you both are safe for intimate contact in general. I suppose you will be safest if you don't ever kiss anyone without knowing their entire health history-- but that does seem like a highly restricted way to live.
First of all, the risk is extremely low. And second, why does that bother you so much? Is it the act itself? Did you think he hadn't been with other partners and that oral sex would not be a part of that? Relax, its completely normal. As long as he remains faithful to you, don't worry. At least he was honest about his past. I believe that its not fair to ask someone details of their past sexual history unless its warranted. It only causes problems in a relationship if there is jealousy involved. Good luck.