3.5 is potty trained, suddenly holding it

Hello, my 3.5-year-old succesfully potty trained himself early last summer and was doing great.  Suddenly over the past week or two he's begun holding it as long as he can possibly manage--at first I had to remind him & convince him to go, now it's a huge argument each time.  And he *can* hold it for hours and hours at a time, half a day and more, and his fluid intake is normal.  But some days I can't manage to get him to use the potty more than twice in 24 hours.

I don't want this to be a control issue, but I'm concerned that holding it for so many hours at a time will cause bladder infections or worse.  I've tried asking him why he doesn't want to go, but all this very verbal child will say is "No, no, no, no, no!"  I've explained that it can make him sick but he doesn't seem to believe me (hasn't made him sick yet, right?).  Any ideas?