I have a PWD that is the love of our life. Unfortunately he has always had trouble with other dogs during food hours if they get too close to his food bowl. We've solved that by never feeding him around other dogs.
Now he has become aggressive with the neighbor PWD coming over for a visit. This new dog is a 6-month old puppy and male as well but we can't figure out why Yogi (our dog) becomes so aggressive around him. We're not sure if it's the pure breed thing, if he's guarding his territory or jealous but it is very disconcerting to see my mellow dog go berserk on a cute little puppy!

Any ideas on how to curb this or whether this is at all normal?

What kind of dog do you have????

Keep them seperated. You can and should expect your dog to NOT be aggressive to humans, their alphas or leaders, but it's too much to expect them to not be worried about their food with their equals, other dogs, whether or not they have their own heirarchy between them. If left to figure it out between themselves, there are bound to be some fights or bites.

PWD = Portugese Water Dog

Has your boy ever seen another puppy before?  He may just be finding the youngster a bit too 'in your face' as puppies can be - and telling him what he can tolerate.  This should be watched very carefully, as the pup may respond, and this will have taught the pup a very bad habit :(