4 month old barking at everyone and everything!

About 7 weeks ago, we adopted a puppy from Battersea who was nervous at first meeting. He is now 4 months old and has gone from doing really well on walks to taking a sudden step back. We really tried hard to socialise him by taking him to parks, shops, cafes etc before he could walk and he did really well, and continued to do well in that first week of walking. However, something seems to have changed. He now refuses to walk with just one person and drags me back to the house until another person can join us. He has also been barking non stop on these walks. I took him to our local pets at home yesterday and instead of being his usual quiet self he was non stop barking at people, dogs and displays! We then took him to training where he was certainly the loudest one there. I tried to distract him with treats and he eventually calmed down but it took a very long while and I’m not sure if distraction is defo the way to go. If anyone had any suggestions on how to help I would be really grateful! Thanks.