I agree, a vet check absolutely necessary.
Many people might not know that if a senior dog has arthritis, the dogs behaviour toward younger "bouncy" dogs can change. It could be that your dog is in some sort of pain and is simply defending himself.
In the meantime take brisk walks with him on-leash, he can still socialise with other dogs, but you decide who he can and cannot sniff. So try and become more aware of other dogs body language; do they look friendly and bouncy, or friendly but calm, or not friendly?
Lots of good books and DVD's out there covering canine body language.
Canine Body Language: A Photographic Guide: Interpreting the Native Language of the Domestic Dog by Brenda Aloff