I just got a second hand Epi LP Custom and my lower strap button came off, I mean, it just came off. Iwant to fix it but I don't know how, please help me.

IMO the hole where the strap button stays is too loose or chewed.
what he says is true, but over complicated. stick in a match stick, snap it off. replace strap bobbin
That same thing happened to mine. I did exactly what the link shows. worked for a good amount of time, then I had to redo it.