Difficult to be sure what would help. Changes in behaviour are fairly normal in young people with autism as they go through puberty, and their processing often seems to get turned on its head.
In case noise is a factor, you could try ear defenders? Having a sensory toy (e.g. fidget cube, soft material in pocket, stress ball) in hand could help.
Does he also feel it's an issue that needs to be resolved. If so, it might be useful reflecting together on the situation (if he doesn't think it needs fixing, this may be less successful...). See if he knows why he gets upset? If you're able to brainstorm any ideas, it might let you structure gentle exposure while reducing what bothers him as much as possible.
E.g. if sound bothers him, then places like libraries or churches/cathedrals can be good exposure to groups of people without the noise of a crowd. If he's worried about people touching him, then making sure he's in the middle/by the wall when you travel would let you protect his personal space.
One thing I think helps some of the kids I work with who have sensory issues is having an exit strategy /knowing there's a way out of the crowd if things get too much. They know where the nearest exits are, and I'll ask if they want to step outside for a bit if we're in a loud/busy room. Sometimes knowing they have a choice is enough for them to settle, but sometimes the break's needed. Either way, they still have that exposure to the stimulus without inducing too much distress.
Can you add any further details? E.g. can your son go to the cinema and sit next to/near other people, or does that trigger his anxiety?
C