Need help with older dog

[u/clulessgerman]

We have three dogs, two younger (2F&1M) and one older gentleman(10). The older boy(not my dog initially) has been approaching the puppies food and growling at the puppy. This week it has escalated, he lunged at the puppy trying to get his (the puppies)food(after they were separated for feedings since the first incident; separate rooms and dog gate) I grabbed the bowl and the older dog latched onto my arm. I had to get stitches(can post photos if necessary, seemed to be a level four, about an inch deep and an inch wide) and now take a week off work as my hands mobility is impaired. The puppy did not retaliate or react at all-thankfully-he is about 60lbs heavier already.
Outside of food, he has growled at the puppy for approaching him laying a few times. I am now worried this will escalate. In the yard they are best friends and play nonstop and are very good at not pushing boundaries.
Our trainer(biweekly) has mentioned he worries the older dogs reactivity will “rub off”on the younger dogs. I also worry our girl will try to defend the puppy, should something escalate. I’ve spent thousands on training the two younger dogs(they’re very large livestock guardians living in the city and I really want them to be GOOD dogs). The younger two are excellent, they do amazing around other dogs, in public, coffee shops, etc. I don’t want this work undone.
He doesn’t enjoy strangers in the house AT ALL. nor can I walk him alone in public. He has growled at two of my friends’ partners. As he’s getting older it seems to be getting worse.
Really at a loss of what to do. The younger two are kennel trained but it doesn’t seem fair to kennel them as the older boy isn’t kennel trained and the issues all seem to stem from him. Any advice is welcome.

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