T.S.
I haven't experienced this with my son, but I thought I'd respond anyway :-)
How does your son do with other changes (non shoe related... changing where he sits at the table, sleeping away from home, new furniture etc)? Does he have some sensory integration issues? It's probably worth thinking about how much this resistance to shoes is a part of his whole personality and how much it's about shoes... you might also talk to his pediatrician if its more pervasive.
Anyway, an idea: have you tried buying a few pairs of shoes in increasing sizes at the same time. As one pair nears replacement time, bring the next one down from the closet. That will give him time ot adjust to them in his own space not in the tramatic shoe store. You could do the same with the seasonal shoes, just buy them before the season and let him have his lead time. By the time it starts raining, maybe he'll have warmed up to the idea of boots :-) Of course you'll have to guess about future sizes a bit, but it doesn't sound like he's letting your get an accurate measurement at the time anyway!
Good luck, mama.
T.