Dog possessive/protective over owner near other dogs

Hello,

I have an issue with a 11mth cocker spaniel crossed with a king charles.

History: introduced into our family at nearly 4 mths old with a 15yr smooth foxy and a 10 yr mixed poodle. He was socalised well as a young dog by visting our other friends, which all have dogs.

Cocker has had 2 operation on a cherry eye at age 4mths also desexed.

Started puppy training at 5mths and we have continue with his training and socalisation with other dogs and are even competing/trailing at the current stage.

My problem is that if we are sitting around other dogs just relaxing, he is starting to growl at other dogs and when i say NO! he starts to want to fight...i have started the quick yank on the lead and turning the other way in which it looks he may respond to, though the problem being in this situation in that being around other dogs they start to look at you really strange, especially if you are at one of these competitions (dog is really cute).

I want to get to the bottom of the problem....should i go to parks where there is a lot of dogs and use this method of correction???

He did start around 5 to 6 mths old to tell the older dog off around food and me. the poodle steps in and problem solve. though he(puppy) stills every so often will tell the older dog around food.

help!