Sounds like you have found a very friendly fella like mine. He demands attention like this too, try running your fingernails over his back - he'll love it. He probably has been hand fed if so make sure you keep your fingers clear of his mouth as he will grab for whatever is in front of his nose. I suggest you contact your local tortoise society for friendly advice and a good diet sheet and they may have a lead on his owners if they are members.
Re leaving out in the garden - provided you have a secure garden he should be able to stay out when the temperature is 10 degrees or above. If in a foxy area I would recommend putting him in the greenhouse or shed overnight. Damp cold days he will need warming up indoors in order to make sure he feeds.