My dog is depressed..Help?!?

Me and my partner just had to get rid of her dog (his best friend) because she is moving back in with her parents and I am as well so we could not keep her. We have been living together for the past 5 months and they have been inseparable since. It has been a little over a week since we found her a good home and my dog (Max, Cavalier King Charles Spaniel) has been sleeping a lot and has been over protective with me. This is not like him and he has never shown any aggression in his 2 year life. Right before we had to give up his best friend (Shivers), we had found a 5 month old puppy wandering and infested with fleas and worms. Of course we couldn't just leave him on a busy road at night with the chance of being hit. When we gave Shivers away, Max has seemed to start disliking the puppy (Oscar). If I try and play with Oscar, Max will come jump on my lap and growl at him. He has also snapped at him. Like I have said before, this is not normal for him. He has never shown any signs of aggression before this and I have had him since he was 4 months old. We aren't keeping Oscar for the obvious reasons, we are finding him a home. Max will barely touch his food now, when before he loved to eat. I don't know what to do for him. If anyone has any suggestions, I would greatly appreciate it. I have a lot of experience with dogs, but I don't know what to do with him.