I too am so sorry to hear of your loss and Chester's mourning.
I've also been through it and it is not easy. You can see from my signature that I was also blessed with two Boxer babies. When I lost my girl from cancer in 2005, my boy mourned BIG time. The two of them were sooo close, in age, and in spirit, it was very hard on my boy. In our case, I was "lucky" (hard to call it that though) that I knew the end was close for my girl, and I knew Dex would take it hard. I started him on Clomicalm several weeks before she passed, but it was still very hard for him.
For us, I made time each and every day to spend special time with Dexter. I'd take him for a ride in the car, or a walk. I'd speak in a happy happy voice (it was hard at times) as often as I could. Special treats, special toys, even though he didn't necessarily want anything to do with them. I gave him as much attention, positive attention, as I possibly could.
I am not suggesting medication, but it may be something you want to ask your vet about if it is really bad. It helped us and is something to think about.
It sounds cliche, but time really will heal the hurt. My suggestion is do not get another dog, unless YOU really want one. Getting a new dog is a big decision.
Again, I'm so sorry for your loss. Give Chester a hug from us! I'm sorry he is so sad... :(
Hugs to you. Hang in there and keep us posted.