How to focus/calm puppy when overstimulated during socialisation?

I've been the proud owner of a 15 week old Eurasier puppy for 3 whole days now. He is the first dog I have owned as an adult. The story of how I ended up with him is long and boring, suffice to say that he was with his breeder until he came to me, and she has done a lot of work with him already. Nevertheless, she was in a rural area and I am in an urban one, so his experiences on walks with me are quite different than they were with her.

In general, he has been taking it like a champ, however I am aware that the socialisation window is rapidly closing. My plan for daily socialisation (i.e. not including our puppy club and obedience courses) is two varied walks and one 'errand' a day, such as a ride on the bus, visit to a dog friendly store or cafe, etc. Today, our errand was a trip to the local pet store just round the corner. I'd actually done that one on Thursday as well, but I needed to exchange something and figured reinforcement couldn't hurt. The first time, he was excited but not overwhelmed and nothing negative happened, so I was really surprised when, this time, he got really upset immediately on entering the store, pulling hard to leave and whining loudly. I honestly had no idea what to do, so I let him go back outside (couldn't really stay where we were as blocking everything). Outside he was still really stressed out and totally unable to focus on me (normally not a problem), even when I crouched down and offered him treats to pay attention to me. Since that clearly wasn't working, I walked him up and down the street till he was calm again, then we re-entered the pet store from the back door and walked through to exit it by the front door (which upset him) at the same pace we were walking outside. He still got tense when we got to the front half of the store and tried to pull to get out (again, no idea why, there was no music, no other dogs, hardly any people), but much better than the first time. We then walked round to the back door again and walked through a little more slowly. He was still stressed, but less than the previous time. He was very interested in a box of chews on the floor, so I gave him one, and that managed to distract him while we stood at the front (the spot where he originally freaked out) and I chatted to the store owner. Every so often he would look up from his chew and seem a bit upset, but nudging it brought him back to it, and by the time we left he walked out normally with it in his mouth. If he hadn't found the chew, I would imagine we would have done a couple more 'laps' of the store before he could be calm.

So, that's how I handled it on the fly, on the theory that 1) his reaction shouldn't be coddled; and 2) not dealing with it could give him a negative association with that store (or stores in general) going forward. However, I'm painfully aware that a novice dog owner's gut reaction isn't exactly very scientific, but now that we're home, I'm really struggling to find proper advice on how to best handle such a situation in the future. My assessment of his reaction was that he was totally overwhelmed in the store for some reason, and unable to shake it off and calm down on his own. I'm especially looking for advice on how to help him deal with this and refocus on me when he's feeling stressed.

Thanks in advance for any advice!

Edited to add that he has all his shots, in case anyone was wondering.

You did everything right. The reaction isn't uncommon. Pups that age tend to suddenly become nervous of things they previously accepted and, of course, we don't always know what they've seen, smelled, felt etc to set off the reaction.

Never force them to accept a situation, but calmly show them there's nothing to worry about.

The chew helped him overcome it so it's worth keeping a couple in your pocket for the next encounter that worries him

Try not to over think things, your gut instinct was right. Maybe in future as Chris says carry a chew in your pocket or get him to focus on a squeaky toy or a bouncy ball or even a small squirty bottle of something smelly. Cubes of dried fish skins or dried tripe will get most dogs attention in a flash, the smellier the better. Their favourites tend to change as they grow.

Many thanks both! It's good to know I handled it right, even if it was more or less accidental.

I will definitely start carrying a small chew around. He's not that interested in treats (even my smelly salmon ones), he'll take them but seems to consider them more of a bonus than a reason for living. However, he loves those deer horn chews. I've read that they're not great for 'boredom chews', as they are very hard, so might be very useful as a 'special treat for being brave' when out and about, as he will be distracted from it quickly enough anyway for me to recover and use for next time.

Also, having noticed his age, he will start losing his teeth in the next week or so so is definitely teething. A sore mouth can make him more sensitive just as it does us when we have small aches and pains.