Given your opinions on pets, you should have named him "Indefensible" after Buffett's G-IV
The dog is likely to be microchipped---this would CYA just in case there's a small probability that he's actually lost and not ditched.
If you're going to keep him and even if you don't, step one would be to go to a vet and get him checked out (heart worm, fleas, parasites, rabies, neutered, etc.). Owners who are irresponsible enough to ditch a pet might not have been too conscientious about that either. The vet would also be able to read the chip or install one if not present.
The city might also require tags/pet tax/chip.
If you're going to rehome, do so quickly, because dogs do get attached to persons.
If you're going to keep, congratulations on your new 5-15 year commitment. The most important I think is to ensure that the dog has at least some training (sit, stay, come) and that you meet its expectations of daily activity/maintenance (read: play or walk). Food you can DIY. I can dig up our recipe but it's mostly rice, chicken, and frozen vegetables (peas and carrots) --- all cooked together in a big pot.
