4 year old wetting his pants several times during the day

My 4 year old, who has been potty trained very well for 2 years has recently started wetting his pants several times during the day. At first we thought it was for attention, then we thought it was because he was playing and just didn't take the time to stop and go to the bathroom. The thing that seemed weird to me was that he was wetting himself in VERY small amounts, SEVERAL times a day. The spot on his underwear is never bigger than a quarter. We had him checks for a urinary tract infection and bladder infection, both were negative. We have tried telling him that if he goes a week without wetting himself we will get him the toy he has really been wanting, that didn't work. This makes me think it's something he REALLY can't control. He is in a really good school and will no longer be able to go if we can not get this under control.  Other than a UTI or bladder infection, what could be causing this? I would love to hear ANY advice, tips, help, etc, etc. We're lost as to what to do next. Thank you ahead of time for any help at all.