Heartbreak

We rescued Izzy from what was sure to be not the best living situation when she was only a couple months old. She ran right into Ace's lap; practically begging us to take her & give her a good home. We already had Belle by that point; she was 2 1/2y and they got along fine, but we hadn't considered getting another dog. At some point, S.W. (who now, again, goes by her middle name) taped a Polaroid of her to our desk. We discussed it and decided she'd be better off with us than in that situation, so we took her. Izzy and Belle grew up nicely together; they played & cuddled, and Izzy would chase after Belle when Belle chased after her throw toys (Izzy never was one for toys). After Belle died, Izzy went into a bit of a funk... We all did. When Izzy's smile came back, we knew it was time to get her another dog. That story is here.
Over the last (almost) three years, Elvis took on the role of Izzy's service dog. As her health has declined, he's told us when she has to pee, when it's time to eat, has cuddled her when she didn't feel well, and much more. He's been very protective of her. So, last Monday, after many months of her having all sorts of trouble, including running blood work & X-ray and putting her on meds, we had to put her down. She could barely walk, couldn't squat to pee without falling down, refused to eat (we tried all sort of foods... nothing worked). We knew it was time. Princess said goodbye to her before she left for school that morning, and Ace & I took her to the vet. She started her life with us in Ace's lap, and she left this world in his lap, too.
Unfortunately, you can't make a dog understand that his best friend is dying. I think he knew she was sick; he had been peeing over her spots outside for a few months and had been by her side more than usual. This wasn't the first time we'd taken her to somewhere without him, so at first, he probably didn't know she wasn't coming back. Those first couple nights, he seemed ok. Night three, however, he barked & whined multiple times througout the night; keeping Ace & I up like we had a newborn. When we put Izzy's blanket in with him, he seemed better. Ace even put Elvis' blanket in there; fluffing it up to take up more space in the huge kennel he's used to sharing.
He's been sleeping better, but still seems depressed. He comes out of his funk a little when he gets to go on walks or car rides, but when we took him to the groomer today, he cried and didn't want to go with her. Maybe it's humanizing him, but I wonder if he thinks we just took Izzy somewhere & abandoned her... and he's next. :( He was, of course, excited when we picked him up, but it's heartwrenching to watch him go through this. I'd like to get another dog, but Ace seems more hesitant... not that he doesn't want another, but it's too soon. He also mentioned "leaving them alone" when/if he goes back to work, but that's not even within the near future, and two together is much better than one lonely dog.
Anyway, we're keeping an eye on him and eventually will get him another dog. For now, Ace is trying to take advantage of any nice days to walk him & work on uninterrupted training, and times he can bring him in the car with him.