Two things occur to me.
First I think being an intact male can often make them more reactive. However my DDs dog is five and an intact male and as chill as they come. He is a big breed.
Second, I read an interesting article about why smaller dogs can be more reactive/territorial than some big dogs. I’ll see if I can explain it well.
If you have a big dog you can’t let it just do what it wants. It will have consequences - might knock over children, sitting on sofa, sitting on laps unasked, could pull you over when pulling on lead.
However small dogs by the nature of their very size in a sense get away with more. I think the case in point was that we tend to let a small dog get on our laps when they want - it’s nice, why wouldn’t we?
But in the dogs mind, it’s calling the shots -not you. That I think makes it more territorial of you. Plus there’s another thing at play, if the dog is in charge then it thinks he’s has to make the decisions over who is a threat and who is not.
The suggestion was to reinforce discipline and manners at home as if he was a much bigger dog. So your permission is needed to to get up on the sofa, bed or your lap. Permission needed before he starts to eat, or go through a door or gate. Makes sure he reliably follows through on his sit and down commands, and ask him to do these at random moments.
If he always looks to you for permission and guidance, then the thinking is that when you meet other dogs and humans, he will look to you first and follow your lead on whether there’s a threat or not.
I don’t know how an accurate this is, I don’t own a dog at the moment, and my last dog was a big breed. But it made sense to me at the time.
Good luck.