Question about collar for feral Ferris

For those of you that have given of yourselves and your homes to ferals, when do you find it reasonable to introduce a collar? I'd like to get one on Ferris as soon as possible so he gets used to it, but don't want to introduce it too soon, either.

Do you think I should wait a couple of weeks, or do you think I should just slip one on him and stand back? (A breakaway, of course!)

I'll be getting a better guestimate on his age later today, but he's very small...no more than 4 pounds maximum.

I've always just put a collar on them when they moved in the house after their spay/neuter and never had any problems with them accepting them. Whenever one has lost a collar, they seem to be happy to have it replaced. I always kind of felt that all those with collars feel a greater sense of belonging. (Of course that could be my imagination ).