Hello, I'm Dr. Deb.
I recently came online and see that your question about Borris hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
First of all, congratulations on giving Borris a well deserved second chance. I simply can't imagine what his life must have been like for the first three years of his life and am so glad that he's found a home where he will be loved and cared for.
As you can just imagine, it will take some time for him to adjust to his new life with you. Towards that end, I do have a few suggestions to help ease the transition into your home. I'll also address the eating/drinking issue.
The fact that he let you touch/pet him at the shelter bodes well for his eventual integration into your home but he has been through a lot of change in a very short period of time, which I know you understand, and most cats abhor change.
The fact that you haven't tried to force him out from under the bureau or tried to force him to accept your attention is excellent. The more you try to engage him if he's not ready, then the longer it might take before he completely loses his anxieties/fears.
This process is unpredictable for each individual cat. For some it only takes days, while for others it can take weeks...and in the worst case situations (which don't happen that often), it can take months.
Hopefully the room in which he's hiding has a door which you can close so that he can have his own space. If not, then I'd try to move him to one which does. Provide food/water/toys and a litterbox and expect that he'll remain in this room for several weeks or perhaps a little longer. Most cats in this situation aren't going to eat if they feel threatened or scared. But, once left alone (especially overnight), you'll find that they will eat when they feel it's safe for them to do so. It wouldn't surprise me if he goes 24 hours without a meal but let's hope that he eats sooner than this.
I'd leave out dry for him to munch/graze on but I'd also put out a small amount of canned food which most cats tend to prefer.
I'd also consider purchase of a product called Feliway which is a natural pheromone specifically designed to reduce feline anxiety and stress. It's available in a diffuser which can be plugged into the way and can be found at many pet/grain stores or purchased online.
I would quietly sit in the room every day for 10-15 minutes at a time (or more)...read a book or quietly talk to him but basically ignore him. If he does start to come out from under the bureau, I wouldn't try to touch or pet him right away. I would just continue to ignore him and let him come to you.
Since there is the history of the previous positive interactions at the shelter, then the odds that Borris will respond to you pretty quickly are pretty high.
Patience is the key here which I think you know. You can't rush the process; you just have to trust that eventually he will warm up to you when he's ready to do so.
As far as introducing him to your other cats, there are a number of excellent articles on the internet about this process with patience also being important. This is also not a process which you want to rush. The following is one such article that I like a lot. http://www.humanesociety.org/animals/cats/tips/introducing_new_cat.html
I hope this helps although, again, my profuse apologies for the delayed reply. Deb