Dido what mammo said. I usd to see how long I could go without going to the bathroom. I was about seven years of age or so. I lost interest after a week or two. Oh yeah, I'd hold it if I was really into something. As adults, we'll say to ourselves "Ok, I'll go to the bathroom now because I do not want to be interrupted when I am busy." I child thinks “I'll go when I have to go..."I playing and having fun...must go now because I can't hold it any longer!!!" I know nothing of bladder infections and can only assume that there would be some indication of discomfort or pain during urination. I really would not use a fever as an indication of a urinary tract infection. In example, where is it written that symptoms of common urinary tract STDs are often accompanied by a fever? No, I've never had one :)
He may just be holding until he can't anymore, and this makes him have to run. Or he may have a bit of an infection going on, check to see if he is running a fever. Sometimes they just don't want to take the time to go to the bathroom until they can no longer hold it. Take care.