What Causes Cold Sores And Fever Blisters After Oral Intercourse?

I ve been with my boyfriend for 1 year and 5 months . We stared have sex 7 months into the relationship. We have oral sex . My concern is he s recently been getting cold sores or fever blisters but I don t get those so I m afraid of what it may be. I recently got tested for HIV , Gonareah , and chlamydia , and came out negative in all. I need an answer .