In pack structure bigger and intact is dominant over smaller and neutered.
Thats a generality of course and all dogs are individuals, but your dog may be responding to the stray dog's size, testosterone odor, and doggy body posture signals you are not seeing indicating the dog's attitude.
Ordinarially your dog would have the advantage of being on home turf and being older but he might not be feeling as healthy and strong as he did when he was younger too.
If you keep the younger dog you may want to get him neutered. But if your older dog is showing obvious fear signs I'd not leave them alone together but would separate them instead.
Hope this helps you!
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