Thrush Or Not?

This is a great source of hostility between my boyfriend and I at the moment.
I have been having bouts of cystitis (bladder infection) for about a year and a half now, ranging from a couple of weeks to three months apart, mainly because I didn't know what it was and I didn't go to a doctor. About six months ago I got it really bad and I went to the doctor, was diagnosed and given medicine. All cleared up. I've only had it once since and not as bad because I have done my research and now know how to prevent it.
My boyfriend has a problem of his own. We don't use condoms and for about a year now he has had a bad "rash" on his penis every month or two which lasts usually a week. He only recently went to the doctor (he was very shy about it and only just built up the courage) and I went along with him because it was the same time as my cystitis and I needed a repeat prescription. He was diagnosed with thrush and has been given some cream for it. I have also been given treatment but I don't know if I should use it.
I know for certain that what I have is cystitis and I have been checked out vaginally and I was given the all clear. My boyfriend's thrush symptoms do not always coincide with my cystitis, in fact his is much more frequent than mine now that I know how to prevent it.
The question is, could I have given my boyfriend thrush once and then it stayed with him? Because I have done a lot of reading about thrush and I can say with certainty that I have not experienced itching or burning and the discharge. I am hesitant to use the treatment because I have read that improper use of it can actually cause vulval damage. My boyfriend is beginning to think it is just my stubborness and refusal to admit I have thrush, and I can't seem to convince him that I am just wanting answers....How come if I have thrush I am not experiencing symptoms?
P.S. Also, would it be okay for us to have sex with a condom when he has symptoms or would it aggravate his condition?