PLEASE HELP I have a lame horse but hes unhanded so I can't check his legs?

Went to see a horse last week and he trotted in the field sound. They dropped him off a week later and he was lame on his front fore in trot in the paddock.

He's been in the stable for 2 days with just 1 hour in the paddock a day. Today he trotted fine for a bit then went lame, then he was lame in walk too. (this is on soft grass and sand. He walked better on the concrete on the way in).

I've been getting him used to being handled but ive only had him 3 days. He still won't let me pick his feet up properly or stand still while I feel for any heat/swelling etc. And he wont trot in hand. So I don't see the point in getting vet/farrier yet when he won't let them check him.

I'm going to put him in the field all day Friday since being in isn't doing any good. Do you think it could be something to do with the stable causing it? Stiffness?

Also could it be because he hasn't had his feet trimmed. He's lived on a mountain so his feet look really good but could this cause lameness on just one leg?

I'm going to get the farrier as soon as he lets me pick all his feet up, and then the vet if he's still not sound but is there anything I can do in the meantime?

Also I'm panicking about it being something serious but if it came on in a week is it more likely to be a pulled muscle, or kick in the field or something?

Sorry for the essay. Any help appreciated!