Help!!! 2 year old is fully potty trained if naked?

My 2 year old boy is fully potty trained if he’s naked. He takes himself to the potty to pee and poop throughout the day and has no accidents in the house. We’re on day 3 with no accidents in the house with him going all on his own without me coaching him. So I tried putting underwear on him today and explained how he still needed to go in the potty and not pee or poop on his new Superman underwear. But as soon as I put underwear on him he pooped in them. We’ve had it happen twice today. So we’re back to running around naked and he’s back to going on the potty when he feels the need to go. So how do I start with underwear going back on? Do I just leave him naked for a week or two? 🤷🏻‍♀️ totally lost here. Because he’s fully trained when he’s running around nude lol. Here’s a picture of my little man.