Potty Training HELP NEEDED!

My two and a half year old began potty training a month ago.  He's a champ at going pee and poop in the potty when we're home and he has no pants on.  The minute I put pants, undies, a pull up, whatever on, it's like he forgets and goes in his pants.  He knows to go in the potty and can verbalize any other want/need he has, but won't tell me when he has to go when he has pants on.  I'm not sure how to get him to tell me that he needs to go when he has pants on.  My older son potty trained in less than a week so this is beyond frustrating to me when I know he's capable - just being stubborn.  Another tidbit is that he's happy to keep playing in wet pants.  Not sure why he pees in the potty when there's nothing to "catch" the pee/poop.  Any help or advice will be much appreciated.  He has to be fully potty trained for preschool in the fall and I'm starting to get worried that he won't be after doing this for a month and still peeing in his pants.  I understand the occassional accident, but it's ALL the time he's in pants.  He hasn't once told me he needed to go when he had pants on.  Thanks!