Welcome to YT !
Bless you for taking in the old guy. The 1st thing you need is to take him to the vet for a health exam & most likely he will need a dental. Get all his belongings if possible as it will make the transition a little easier & be prepared that he may be depressed for a bit as he will be missing his owner , if this is the case just be patient with him & give lots of LOVE. Also even if he's totally house broke he may have some accidents in your home as it's all new & confusing to him so when you bring him into your home , go straight out the back door before letting him down in your home & when he does his business out side tell him GOOD BOY go potty. Take him out several times for the 1st couple of days so he learns his yard territory & where to potty & be sure you watch closely for him telling you he needs to go out. If he only has a collar, buy him a harness as collars are not good for small dogs trachea.