What can I do for my aggressive dog?

I have a cattle dog, beagle mix, he is about 2 years old. He gets a little aggressive over his food with my other dogs. It basically started with that. Now when people come into my house he nips at their legs trying to get them out. Nothing serious yet, and he doesn't leave any marks on them but I fear he will soon, and I don't want the drama of a dog bite. In fact I believe aggressive dogs should be put down. Just today he was growling at my roommates chihuahua mix through the gate that divides the back yard and the patio, so my roommate is all freaked out. Is putting him down my only option? I have tried most of the training for it that I can do myself. I'm not going to spend a ton on some self-proclaimed dog trainer, since there's no licensing of them in my state, and I don't want to dump him on some person to try and fix him either. I think a shelter would put him down anyway, and they are overcrowded either way. He is neutered by the way. Anyway what can I do? anything?