We need some advice/help, please...

Hi, we're having some problems with our cat, Emery. He's 16 months old (so a year and 4 months), and he's the cuddliest, most sweetest cat you can meet. He loves to talk and cuddle in blankets. Yesterday, we noticed a change in his behavior. 

My sister (his owner) had her birthday party, and her friend, Bryan, came over. Emery saw him and immediately freaked out. His eyes were huge, round, pupils were giant, and he wouldn't let anyone hold him. He wouldn't stay in the same room as anyone, he ran and cowered in the basement. We have absolutely NO idea why he acted this way, he usually likes people. We don't know if it is because Bryan was a new person to him, or not. Emery has met him at least 3 times before, but he wasn't used to him. 

Emery is still acting this way today, just not as severe. My brother got a drastic haircut (shaved beard, cut off long hair to almost a buzz cut) and Emery won't even go near him. 

Is this a symptom of social anxiety, anxiety from change, fear of new people, or what?

This is really weird for him to act this way.

We have 2 other cats the same age as him and they didn't act this way towards Bryan OR my brother's haircut. Emery is the only one who showed fear from it. 

He's going to the vet on Friday to be neutered, so maybe it'll calm him, but we're afraid that he's going to act this way toward the veterinarians because he does not know them. We'll probably ask them about this situation as well, but for now, I thought that I would get your guys' opinions.

Thanks.

Sixteen months is a little late for neutering, but it's sure better than never.

Emory may well have had a bad experience before your sister got him.  Cats survive on reactions to little clues, so they remember very well anything that may have led to a bad experience in the past. 

To put it in perspective, our Snoopy would run and hide any time we got out a broom, or any time he saw me wearing a hat.  Draw your own conclusions.

We haven't had the finances to neuter him until now. :/

And we got Emery when he was little over 2 months old, he was a farm kitty. He never had any experiences like that.