He may have hit a secondary fear period... mine did at about the same time. She was super confident with other dogs, and generally just not bothered with other humans until she hit puberty... then overnight she became scared of young men with dark hair or in hoodies, then bigger dogs, then not keen on anyone in general except us and a few selective dogs she knew well.
a) Get a positive force free behaviourist in. If you join ReactiveDogsUK on FB they will reccommend a trainer in your area.
b) Do not force your dog to socialise. It doesn't help and generally makes everything worse (I'm not saying you do btw..but just in case.. avoid the dog heavy parks etc.. it won't help him feel more confident.. it would be like putting me in a room full of large slugs and saying ' see it's fine'!.. I'd fight you to get away!)
c) Until you have had professional advice and support... be his advocate. He's reacting because for whatever reason (and there was no reason for my girl either), he's scared! Mine does NOT like to be approached by strangers, but she's a rare breed , fluffy and beautiful and it seems everyone thinks they have the right to try and stroke her... which makes her panic and bark. So I have a yellow dog 'anxious' harness and I call out 'sorry I'm just moving away because my dog is nervous'
d) Personal preference... I take my girl out into the countryside for long off lead walks where I am less likely to see a soul, at least a few times a week...or on a long line if she's likely to see deer . If she has been upset by other dogs, I give her 'rest days' where we nip out for a poo but that's it... to let her cortisol levels subside. Sometimes NOT walking your scared dog is the single best thing you can do. Scatter food at home, have fun games.. but let him have a break from what scares him.
Sorry, that's long and I'm no expert, but as someone with a wonderful, but nervous dog, I've spent a fair bit of time learning, and still am.
Also.. contrary to popular belief, quite a lot of dogs, once they reach adulthood.. don't actually like other dogs much at all! It took me a long time to realise that I thought she should socialise. In fact mine, like many others, just wants to be with her family and merely tolerates a few dogs she knows well.
We are 3 years down the line, and in general my girl is much better than she was, but within clear limits. I don't do busy dog parks, I don't go to places which I KNOW will be hard for her, and when I walk her I am in charge... I try to read her body language ..if I see another dog on lead, she is beside me, never in front, so we can do an about turn, I can redirect her quickly so she can't fixate on the other dog.. it's amazing how that helps!