evilamc...that is what I refer to as "show ring mentality".
Basically, they have show horses and you do not, period.
So your needs are different and approaches to an animals health and comfort level will and must be different.
They body clipped their horses of natures intended defense from the cold where you have left yours fuzzy and warm...so they must blanket and heavily too unless they plan to body clip again and you do not need to deal with that.
Their horse can't go out in just about any weather without some protection except on warm sunny days for short periods of time. Their horses have no way to puff their coat for warmth & weather protection as yours does.
I don't know if you will be able to convey that your horse is better prepared for any weather condition with what Mother Nature has provided than theirs who now are dependent upon their humans for their comfort levels.
You can talk till blue in the face but some will not listen nor want to learn that there are other ways of doing things.
It sounds more like this is "a kid" with this attitude not an adult.... some kids and adults think they know it all about horses when that is so the opposite...you learn everyday if you are truthful when dealing with horses.
Regardless, you are a "backyard rider" in a "show barn" atmosphere it sounds like... that is not a put down at all.
It is just a different approach to doing certain things with our horses, in this case blanketing yes or no.
Be firm with them on what you want. This is your horse and you are responsible for him, his bills and health concerns.
Actually, if you bent to their way of thinking now after he has grown his heavy winter coat for protection...he would probably get sick from over-heating or be miserable.
Be firm in your wants for your horse... you decide what and when the blanket makes a appearance, he is your horse not theirs.
I would bet you are going to find other issues you don't agree upon with your horses.
You may have to agree to disagree on some issues or look for another barn to keep the horse at. It is supposed to be fun owning & riding a horse, not to be challenged all the time on the way of doing something...
Just be firm in what you want.... I have had the "show barn" mentality and that of the backyard owner & rider....different approaches and neither is right or wrong, but with a different end goal in sight you need flexibility...
I found it easier to not mix the two elements in one barn only because the way of doing things could be different and I got tired of the static from others.... you might come to find the same thing true.
One thing I have never changed though is my "if in doubt, call the vet"... I don't leave things like that to a watch, wait and see like others in any discipline/style of riding activity. I would rather a vet bill and peace of mind than "What did I do" at my horses expense.
Good luck...and ENJOY your horse!