Clam yes it is exactly right for him to start being frightened about now. He's just beginning to become aware of himself and the dangers around him.
I'm sure you've done everything you can to socialise him, so don't take it as a sign you've not done enough.
The way to deal with it is ignore it. Although our natural instinct is to soothe, cuddle, stroke and reassure don't do it with a dog! They will take any sign you give as positive affirmation for what they've just done, ie they'll think their reaction was the right one and carry on doing it.
If you have to speak, keep your tone as a jolly hockey sticks, carry on regardless happy voice and keep on going. Past the door of Pets @Home as you did, past the ponies and the big dog. You're aiming to teach him that they are things to just deal with; to ignore and carry on.
I can't emphasise enough how important it is not to say 'Oh, it's alright Fido, don't worry about it . Don't even make eye contact if you can avoid it.
Go for the sing song 'silly, silly billy - come on and jolly along the lovely ponies' and keep on going. Try and keep attention on you with treats and foolishness.
Honestly, you'll feel like an absolute tit - and probably a quite unkind but it'll equip your boy to be a happy, confident adult in the end.