Thank you thank you; I appreciate any and all advice! As the day has gone on, things are really improving! He's such a love, I'm half hoping he won't belong to this lady

. Well, actually I was half hoping that to begin with. He doesn't seem to be marking in the house (from what I've found anyhow) anymore and he's stopped humping Max!
I am going to take them both out for a walk in a bit when it's a little cooler outside (it's almost 90 right now).
I have another question though; the little guy is so thin, and I'm sure it's from not eating for awhile. The girl who found him was feeding him Pedigree dry food only, and I went and got a small bag of it just so I wasn't changing everything in his entire life all in one day. I also gave him some Hill's I/D b/c I feed Max wet food too, and I would have felt bad not having something for him (and Max is on a low calorie/low fat diet, not I/D, I just keep a can on hand for emergencies).
Anyhow, little guy is so thin, you can feel every vertebrae in his spine, his haunches and his ribs. How often should I feed him and how much do you think? He absolutely WOLFS down any and all food, except for some peanut butter "pupcorn" treats I just bought. I hate to leave food down for him b/c I know Max will scarf it up (it's like they're having a race when I put the food bowls down, LOL) along with his own.
So, should I give him a certain amount like several times a day?
I'm attaching a couple of photos that the hopeful owner sent me and a couple that I took today (they're terrible, I know), what do you all think? Doesn't it look a lot like the same dog? Oh...his real coloring on his body is very silvery/blue.....it looks darker in my photos. The first two are my photos; the others are the lost dog.