Whoa! You never took him to get microchipped scanned? Think lost not abandoned (stray)! My princess takes a bath goes outside and comes in half matted-f she were out and not brushed for two days I know she'd be dreaded and filthy.
Recovery Tips: Think Lost, Not Stray | Missing Pet Partnership
I think what you are doing is noble and out of kindness but consider this for a moment please:
You may be the reason someone is distressed over their missing furbaby!
Any veterinary office should scan him for free for a microchip...post fliers, craigslist, ebay classifieds...you can report a found dog to a shelter (animal control) and let them know you will foster...until you do any of this you can easily be considered a dog thief.
I just feel so bad cnsidering all the people that post their lost baby on here-and judging by his condition...he could have been a rescue someone adopted several years ago! My pup mill pup rescue was adopted at 5 months-he has issue for the rest of his life due to his treatment prior to me...and reverts really easily imagine how he would have been if I had gotten him when he was say 3 years old and then lost him what he was 6...he may look a lot like the guy you found.