Ok, guess I will put my 2 cents in, if you dont mind.
I worked in a local rescue for many years here in NM. I was the training and surrender coordinator.
Here are a few tips for you:
1. I would talk with the rescue and tell them that you are a little nervous with taking on this project, tell them the truth. They will most likely offer the foster to adopt, although some rescues do not do that.
2. Scared dogs will not show their true personalities for many weeks, so if you are willing to try, please know that to be fair to the dog, you should make the committment to keep him at the very least 2-3 months. Usually that is how long it takes to get a dog adopted.
3. It is very rewarding to watch a scared animal come out of his shell with your love and care

4. You do not want to baby this dog, he does not need sympathy, what he needs most is a calm and safe environment with clear, concise rules. Firm and fair is the best thing for him.
5. You will need to work on his confidence level most especially, and his arousal level. Dogs that are used to being afraid have a very low threshold with respect to their arousal levels, you just need to focus his training on raising that level.
6. There are places you can go for information and books that will help. I strongly recommend the following two, get them as soon as possible they will help alot with the initial transition
How to be the Leader of the Pack by Dr. Patricia McConnell
Cautious Canine by Dr. Patricia McConnell
both books will help you with his integration into the home.
I have worked with many frightened animals, and I would say about 99% of the time they successfully integrate into the new family, and the hardest thing is forgiving yourself for the other 1%. If the rescue is willing to place him at this time, they are pretty confident of him being in that 99% group. I know the rescues in PA, if it is the Boxer Rescue I know, then you are in good hands.
Good Luck with your decision....