Well, this may be the way he actually is -- most puppies do not bark much, and dogs tend to start barking around adolescence, which is right at his age. Some dogs are barkers, and some are not, no matter how much training you do with them (Jaspar is a 'talker' in certain situations and this has been virtually impossible to stop). Barking is a natural way for a dog to express emotions, interest, excitement -- just as we talk. So it is perfectly normal for him to bark. It is just that that activity sometimes needs to be managed by us.
Has he been given any formal training? If not he is well into the point where he would benefit from an obedience class as this enables dogs to be less excited in new situations with stimulation around, and also teaches a dog self control and to pay attention to you. People often think teaching from a book at home is enough -- it really truly isn't. A dog needs to learn in a structured way and with distractions; and also hugely benefits from getting used to and meeting lots of different dogs and people. So if you haven't yet I'd immediately look for a rewards- based class in your area.
Exercise is also one possible aspect -- he needs to be getting a considerable amount of exercise now, not just little puppy outings as when younger. Think teenage boy -- from now on he really needs a good hour daily of *active* exercise, not simply a walk around the block but something longer and brisk, or swimming, or fetch, or agility, or something like that. Something that works his brain as well is good, which is where an obedience class can come in with a 15 minute to half hour of home training daily to work that brain.
Neutering at this age also tends to help calm boys considerably, if that hasn't been done. If he hasn't been neutered, the flood of testosterone in his system right now can make him ultra-hyper.
Training though, and from now on, is a key aspect of having a dog that you want to have around. He has no idea that barking isn't OK. You have to find a way to work positively to give him the outdoor manners you'd prefer. This takes a lot of time and commitment -- people always underestimate the dedication and time training takes and the earlier you start, the easier it is (I recommend from the day you get them home to lay down fun and easy basics like sit, and ask for a sit before they get food, for example -- a good way to teach the 'no free lunch' philosophy and get a dog eager to look to you for direction
Keep it light and easy with no big expectations, just like getting toddlers to say please and thank you. You just give them easy things to do, don;t overtrain, and keep expectations in line with their small amount of ability when they are very small). A dog needs daily or many weekly practice sessions at home, too -- like kids and adult humans, use it or lose it!! Adult dogs are far more challenging in many ways than puppies, where training is limited to housetraining generally. As dogs grow into adults they need a lot of time and dedication by us just as children do, to be the dog they can be.
All that said: not all dogs are great companions for taking to shops and some will always be barkers and get overexcited. If that is the case, better to leave him at home and do the shopping without him along. I'd at least be doing that til you slowly work towards training him to be able to go someplace without getting overexcited. This can take many months of work.