Boston terrier puppy barking growling at strangers/dogs

Hi everyone, I’m looking for a little support and perspective. I brought home my 5-month-old Boston Terrier, Loki, a couple weeks ago. At the breeder’s when we first met him he was super social — wiggly, happy, and loved everyone. He jumped right into our laps and gave us kisses, no signs of fear. But since coming home, he’s been barking and growling at strangers and new situations.

We have only had a couple guests come over, he hasn't been able to get comfortable with them yet. No pets, no wiggles. He did take treats still, but still on high alert and initial reactions were barking/growling.

I think the transition plus hitting a fear period has made him nervous about people and new things. I’ve been working on going slow — keeping things calm, doing short car rides, and letting him observe people from a distance with treats — but sometimes it’s discouraging when he reacts again after a few quiet days, or if someone walking at the end of our driveway while he goes potty-barks, growls, hackles up.

We also had to do a wellness check within 5 days for the breeders contract, it went super awfully. The vet said that this is not normal for a puppy his age, it definitely counts as a medical illness 😔

At home he’s the sweetest boy — cuddly, playful, and affectionate with me and my boyfriend. I just really want to help him feel safe with the world again.

I am worried he wont be the go everywhere dog I had dreamed him to be, like our last Boston, if he can't get out of this phase. I am also worried I could either go too slow or too fast, I want to expose and socialize. But I also dont want to flood or overdo it.

Has anyone else gone through this with a 5-month-old puppy who suddenly became this fearful after leaving the breeder? How long did it take before your pup started feeling confident again?

Side note: he is kennel trained. Sleeps throughout the night and we have left the house for up to 2 hours by now. He fortunately has not whined (I have camera set up). So he understands kennel and alone time means nappy and relax time.

Any reassurance or tips and support from people who’ve would mean a lot 💚