Resource Guarding?

Question
I have a 10 month old large crossbreed male pup. I got him as a starved rescue at around 2 months old, and he has been neutered at 5 months, however I suspect he has quite a bit of testosterone still, for various reasons such as marking over females' urine since almost 4 months, etc.

As a youngster (first month or so I had him), he had some food aggression issues, and I've worked with him every meal time and he will know more than happily and very relaxed accept hands in his food and give up his bowl when asked.

I, or any human really, can touch his food or treats or toys, take it away, give it back, play with it, whatever, BUT he does guard treats and toys especially (as he never gets fed around other dogs really) around other dogs.

He has been really well socialized and is very mild mannered around all types of dogs, and he has his basic obedience cues mostly under control, but if he does take a liking in something, whether it be his or some other dog's, he will start really guarding it the longer he is left with it around other dogs. He does not snap at the dogs, but he will growl and show teeth pretty intensely if they come close to it. Even in that state, he is more than happy to give it up to a human luckily, BUT though he will at home alone 'drop it' from a distance, he will not do so when there's a dog threatening it and you have to go close and tell him to leave.

When another dog HAS something that he likes, he will never go and take it from them, he will keep watching and sometimes following and waiting until they drop and go do something else, then he'll go get it.

I would like to know if there's any exercises I can maybe do with him, that might lessen this behaviour rather than let it escalate or become a massive problem.

Answer
It is a symptom of territory aggression. And honestly, since you made great head-way with the food aggression, I would step up the game and reinforce this habit as well.

Whenever my dog gets a little possessive, I remove whatever is motivating her to react and that's the end of the story until she can earn her reward the next time around. And this cycle would continue until she figures it out that this behavior does not get her what she wants.

Also, during these times when he is "preying" other dogs when they have something he likes, I would use redirection with him to a) distract him from preying and b) to give him something else he can be playing with or getting attention.

If you didn't know, food/treat/toy aggression stems from having to fight for food, etc. when part of the litter. We just need to make him realize that it's okay to share and it's not worth fighting over and there are other things that he could be doing with his time.