Ok bees, just need some opinions here. I feel like my LO is between sizes and it stinks!

My LO is 2, he's tall 36 inches and about 30 lbs. Since he hasn't been in pants since June I went out and got him a bunch of stuff at Gap over labor day when it was 50% off, etc. but for some silly reason I bought him 3T -- thinking that 2T and 18-24 months were basically the same and his 18-24 month stuff was way too small. Well, the 3T pants are super, duper long and has major saggy butt since he's kind of on the "thin" side. SO, do I return all of it and get 2T even though he is on the end of the size chart? Do I just hold onto it and get him other 2T stuff for a few months? Have him wear it all huge? (I don't really like doing that if the butts are saggy). His 2T stuff that he does have fits him perfectly right now... but how long does that last at this age? Does Gap stuff run big and I should just get him a different brand?

Why can't they make 2.5T?