"... what if he gets lost and hurt and a stranger does take him to a vet, but not a vet I've seen before?"

I don't know if you can add notes to your microchip service (usually AVID or HomeAgain), but you could call to ask. It might be easier to have info engraved on a steel collar tag (NOT aluminum or other soft metal that could wear off) that you're good for the bill... "injury bills guaranteed" or something? That also might be an impetus for a kind finder that is hesitating taking the dog in, in fear they might be stuck with a big bill. And there are still a few vet clinics that haven't invested in the the chip scanners, not many, but some.

Another way might be to have a out-of-area relative's (same last name, preferably) phone number on the tag for alternate contact, and brief them on what to say. Have them guarantee payment, even though you'll be the one to pay.

Also make sure you understand how the chip system functions, and what you should do if he is lost or stolen. I know you can call AVID and have the dog's number flagged LOST or STOLEN, but I don't know if anything else.

Sue