I would be surprised if it was an allergy, but that does not mean that he still gets an reaction like one. My boyfriend can not finger me, because my fluid is two acidic and it hurts his cuticles, he can give me oral but not very long because he will get a rash on his face if he has just shaved. I under stand this and I am ok with it, if there was something about his body i could not do like oral sex, he would under stand as well.
and even if this is your bf way of telling you he just does not want to do it, that is his choice, just as if you did not want to give him oral would be your choice. it should not be something you have a break up over.
I've heard of gag reflexes and, personal dislikes to taste and odor... but, never allergies.
Maybe it's possible... or, maybe saying that it's an allergy is really his way of saying that he just doesn't like doing it.
While I have to say that I doubt this... It is possible in theory.
I have known of several women on the converse side who had no problem with oral sex in its conclusion but intra-vaginally would have reactions to seminal fluid. The simple truth is different peoples bodies are sensitive to different things. That being the case it also doesn't mean he will react to YOUR body chemistry the same as he did with another woman will probably, but not necessarily cary over largely according to diet and alkalinity.