Potty training help

My 3.5 year old starts school next year and keeps pooing in his pants. Hes mastered using the potty and toilet for wees but not poos. Most of the time he seems to know he needs to poo and will be offered the potty/toilet but says he doesn't need to go then suddenly he's messed his pants. I'm getting really worried now about how he will be, his older brother by this age was completely potty trained during the day and almost dry at night. I've tried leaving pottys in different rooms easy to access, lots of praise, frequent asking. It happens at nursery too. Am I worrying too soon? Any suggestions on what might help thanks