Agree with those that say the reason Alexander is so popular on here is that it is a timeless classic so won’t give away age or social background; is very international so has few spelling or pronunciation problems and has lots of diminutive options so he can make it his own (and has lots of interesting historical namesakes as a bonus).
Names like William, Joseph and Thomas come very close, except that there are not as many diminutives for some of those; as do Edward or James nearly but they are not quite as international. Other timeless names in the UK include George, Daniel, David, Michael etc but again they might not tick as many of the boxes as Alexander.
The problem is that a name that has all these great attributes by its very nature is going to be popular - because it makes them can’t-go-wrong options and an easier choice for parents.
Luckily even a “popular” name is used far less than it used to be even 30 years ago - you may know plenty of adult Alexanders and there may be others over the whole school but yours would still likely be the only one in a class these days.
Or if popularity really bothers you that much then there are plenty of similar style name to Alexander, they just won’t necessarily have every one of the other benefits (such as timelessness etc). I would say Nicholas, Laurence, Maximilian and Benedict immediately sprang to mind to have similar style to Alexander imo - as all are well known, longer length names with classical origins.
ETA: You could also always use an unusual nickname for Alexander too rather the usual Alex - personally I like Lex, Xand or even Sasha.