Should I put my dog to sleep?

I have a 6 year old chocolate lab that has shown aggression since he was a puppy. We socialized him very well (visits to Petsmart, training cles, dog day camp, walks in the park). We stopped the day camp when he became too large to control him from lunging, barking and growling at people coming in with their dogs. He is 140 lbs and not overweight - just a freakishly large lab. He barks and growls at neighbors he sees every single day. He barks and growls when ANYONE, even us, enters or leaves the house. He is extremely affectionate with our family and my 8 month old grandson. I have new neighbors moving in with three small kids and I'm worried, and embarred, that he will bark/growl at them. He's never bitten anyone, but how can I be 100% certain he won't? This aggression, coupled with the fact that he was diagnosed with diabetes in March and has to get two insulin shots a day, glucose checks a few times a week, and $100 per month for the special food, makes me consider putting him down. The problem is I love him so much. He is so funny and affectionate with us. I also have a 7 year old mastiff who would be lost without his buddy. We have tried trainers/training methods and nothing has helped. Any thoughts or suggestions would be greatly appreciated.