the Polish lad in dds class is Kacper, (pronounced Casper)
The point is, that in reception we all went Oh you spell it THAT way, and ever since we all, without thinking just write Kacper. He doesn't meet hundreds of new people every day at school, it is the same class of kids who all know it.
In the same way that people for centuries have gone Stephen or Steven and Maya or Maia and Katherine or Catherine and Claire or Clare.
It really isn't a problem. Yes of course he will get Alex and Max occasionally, but you know what, I have a very ordinary name beginning with R, let's say Ruth. Ever since I was small people have called me Rachel by mistake. I even have a set of certificates from the local poetry festival with Rachel on them, as my own teacher 'forgot' when she filled in the form.
My son's name has more than one spelling, except the more common one is WAY more common, about 90% spelt his way. He got a certificate spelt the other way, despite the form being filled in correctly.
It happens, it isn't unique to non English spellings. When I look at my kids friends very few have names that I would be 100% confident of spelling right without asking, and they aren't that unusual.