It’s not all that unusual for a 5 year old to have trouble with transitions but, as you know, change is part of life and it’s your job to help him learn to manage it.
I suggest you try some “pre-teaching.” If he has a favorite stuffed animal, see if you can find or make a season-appropriate outfit for it. Talk to your son about how it’s that time of year again when everyone has to dress for warm weather. Ask him how to talk to his animal so that it will be comfortable with warm weather clothes. Then introduce his own warm weather outfits gradually. Maybe first just ask him to help you get them out of storage and put them away. (If they are already in his bureau, spend an hour with him sorting out what he has outgrown from things he might want to keep.) Don’t even talk about putting them on. Just work on sorting together.
On another day, you could ask him which warm weather clothes are his favorites and which ones he might want to use first. Then see if he will help you lay out appropriate clothes for the next day. Some kids have fun making an invisible “person” by laying clothes out on the floor.
Not knowing your son, I can’t be more specific but you get the idea. Essentially, the goal is to give him a long and gradual transition instead of an abrupt one. It’s much more likely to work if you keep it light and make it fun. Once you’ve done this kind of thing successfully a few times, he will get more comfortable with making changes.
I wish you well.