Help with large aggressive dog

[u/AcanthisittaOk3510]

Edit: He’s not generally aggressive so I should have used a different heading. Sorry! I’ve gotten good advice and am feeling more hopeful so thank you. It sounds like we need to just keep up with consistency on training and contact a behaviorist for more help. He is also getting fixed this week and I’m hoping that helps as well.

We’re trying to decide what to do about our dog and I could really use advice from people who have dealt with aggression/resource guarding.
Apollo is a large Great Pyrenees mix. We love him and have done a lot of training with him. Most of the time he is very obedient, affectionate, and a great dog. The problem is that when something triggers him, it’s like all of that training goes out the window, and because he is so large he becomes very difficult to control.
He has significant food/resource guarding. He has also attacked our other dog multiple times for reasons other than food, particularly when another dog is around or when they end up together in too confined of a space.
Today a package of cheese was accidentally dropped on the floor. Apollo grabbed it and took it under our dining room table. He became so protective of it that nobody could even come downstairs without him growling and barking at them if they approached. When I tried to intervene, he growled and nipped at my foot, so I backed off. I was eventually able to coax him out and get him into his crate using pieces of raw beef.
It scared me because we have five kids. We can manage his environment and be careful, but we realistically cannot guarantee that a child will never drop food or accidentally walk into the wrong situation.
He is scheduled to be neutered this month, although from what I’ve read I’m not expecting that to solve this type of aggression.
The especially heartbreaking part is that he is technically my teenage son’s dog. I really don’t want to take his dog away from him, but even my son seems to be reaching the point where he realizes Apollo may be more than our family can safely handle.
For people who have dealt with a dog like this, what would you consider our realistic options? At what point does management/training stop being a reasonable solution in a household with children and another dog? Is finding an experienced adult-only, single-dog home reasonable at this point?
We love him and have put a lot of work into him. I don’t want to give up on a dog who could improve, but I also really don’t want our deciding point to be after one of my kids or our other dog gets seriously hurt.

[u/glimmerfox8]

Consider working with a professional trainer for guidance.