Wow, you are a very dedicated dog owner. =D I applaud you for your rescuing and fostering, which is something I plan to do in the future.
Anyway, it sounds like (as you said) that he's intimidated. I would let them have there space for a few days, and then try getting them more properly introduced.
Something I would suggest would be to find a toy both dogs really like. Weather it's a tennis ball, rawhide chew, greeny, etc. Once you've done that try letting them play with it together.
You can also find an activity they both like. Maybe they both like to go for walks, or enjoy a special place like Petco. Just find things that both dogs like and let them do it together. =D
Anyway, it sounds like (as you said) that he's intimidated. I would let them have there space for a few days, and then try getting them more properly introduced.
Something I would suggest would be to find a toy both dogs really like. Weather it's a tennis ball, rawhide chew, greeny, etc. Once you've done that try letting them play with it together.
You can also find an activity they both like. Maybe they both like to go for walks, or enjoy a special place like Petco. Just find things that both dogs like and let them do it together. =D