My kids have picked out their own clothing since they were 3-4. When they were 3-4 I would often take along extra clothing if I felt their choices were inappropriate, but 99% of the time they were fine. I will ask that they bring a light jacket if the weather is under about 55 degrees and they're wearing short sleeves. They usually follow along.
But the reason they usually follow along is that they've experienced the consequences of not dressing appropriately. So, they've been hot when I suggested shorts and they declined. They've been cold when I suggested long pants/long sleeves and they wore shorts and a short sleeved shirt. Now, we live in a relatively mild climate (rarely under 40 degrees, rarely over 85). But I find spring and fall were the times when they had to LEARN with their own bodies what they needed to wear. Remember they're still very physical learners at this age. There's nothing like experiencing cold legs to realize, "oh, mom was right, pants were a better idea." (they'll never say that mind you.)
At about age 3, my ds started to read the weather page each morning. At age 6, he'd start using that information so he could tailor what he wore. Dd is 7 and her wardrobe is still hit or miss. Today she wore pink shorts, a tie dye t-shirt, and over that a long sleeved t-shirt that was black with polka dots. She added blue snowman socks and pink shoes. She's not exactly a fashion plate, but it meets her needs -- it's cool in the morning and she wants the extra warmth on her upper body. When it gets hot, she can take off the long sleeved shirt. OK, none of it really matches. Either she'll learn to care or she'll grow up with a quirky sense of style.
All of this long message is my way of saying: Picking out clothing is a perfect way to learn natural consequences of your action, at very little risk to your child. Unless it's below freezing, the biggest risk is discomfort.
Instead of telling him what to wear, I'd help him see what the weather would be like and have HIM choose. Then you can talk about how he felt during the day. Remember, kids move a lot more than we do. They're often much warmer than we are. Even if it's cool, he might be uncomfortable at recess in pants.
If YOU take pride in what they're wearing, then yes, you're doing this for you and not them.