Since he's used to the temps inside, I would wait till the weather warms up at night. You said he roosts outside, but he's got a secure coop to go to at night, right? Having birds that stay outside is a recipe for owls/coons/possums to get a free meal. That's how I've lost some of my birds in the past, when they went broody on a nest, but these were guineas.

I add chickens back into a flock at night when the chickens wake up in the morning they seem to not notice the new chicken. I have also added baby chicks under a broody at night and in the morning she didn't seem to notice or care that she had more chicks than the previous day. When the weather is a bit warmer you might try putting him back on his roost at night.
Suellen
I raise Delaware Heritage breed chickens