Really, not many people would commit to taking a little guy like Teddy. You truly are an angel for him.
With most of his teeth missing and it looking like he hasn't been to a vet in who knows how long, outside of your friend taking him, he could have a sore mouth and it might just be best to keep him on wet, or canned food. I'd also try several meals a day. If you work, you could feed him in the am, again upon returning home and another later in the evening. I'd also supplement his diet with a fish oil capsule, if he will take it or eat it on his food, as it will benefit is skin, joints and heart.
I'm guessing he could be a bit older than was estimated. Many vets try treating a condition with antibiotics first, it seems, then start looking at other problems. Your vet might want to do more tests and if your friend didn't have bloodwork done, it would certainly be worth doing.
Also, until he is healthy, say NO to immunizations of any kind. You can assure your vet you will get them done when his health is better, but his tiny little system doesn't need that stress right now.
He is so lucky to have you and I'm betting you're going to feel blessed to have him!