It's HUGELY popular, thousands upon thousands of people go. It's full of p*ssed up Aussies and Kiwis on 'the OE' (overseas experience). It's massively over-crowded, dirty, noisy and can be dangerous - broken glass all over the place, muggings, fights, etc etc. There are always many alcohol related injuries, some serious, after gone wrong high jinx - statue diving for instance.
The bull run (encierro) itself is also way too crowded. You're more at risk of being injured in a stampede of scared people than of being gored! Some that run with the bulls (mostly steer actually) are invariably the same pissed up folk you've been avoiding during the night... but they're now sleepless into the bargain. A recipe for injury imho.
I stopped going 20+ years ago because it was getting that there were too many people!
In all seriousness, if your lad cannot handle crowds then he shouldn't go. It really is very very popular.