I hate stabling my horse, but if he was lame and I didn't know why, I would err on the side of caution and keep him in until I could get the vet the next day. There are too many unknowns, plus it's going to be windy tonight and I wouldn't want to risk him running around in the wind and making anything worse.
I'm not experienced in lameness and would be clueless about what to do, but I know that my knowledgeable and trusted YO would advise to keep in to be on the safe side.
I have messaged my farrier to ask them to look at it tomorrow am, it does sound like it may be an abscess.
Apparently its not uncommon for a horse with a abscess to be lame, come sound for a few days and then come in with worse lameness that gets much worse.