My dog is now aggressive toward dogs? He became aggressive after getting attacked by a rott...Help!?

My dog is 5 yrs old. He has always been very good with dogs. Last month he was attacked by a huge rottweiler. I had to keep him seperated from my other dog (all dogs) while he healed as he required stitches ect. Now that I have tried to get him around other dogs he is very aggressive & defensive. How can I help him to get back to himself?