Well when I introduced my puppy to my dog (at the time my dog was 6 years old and my puppy was half a month old) I gave her LOTS of attention and had them play a little bit together when they were in the same room.But then again my dog is pretty calm and didn't really bark or do anything when they first met.
If your 5 month old has ever been aggressive,then put him on a leash and have someone standing by to pick up the puppy in case it lunges.
Was it playful barking or attack barking?If its playful,than let the dog bark and try to soothe the dog,when it calms down try to bring the puppy closer so they can sniff each other.
If its the other kind of barking,than do the same but on a tight leash.