I don’t have any knowledge about the types of courses you’ve mentioned and can only speak for social sciences …
Many of the English language programmes will be relatively new and the institutions may not have built up the teaching expertise, reputation in the field, and may still be in the process of trial and error.
It may not be such a problem for the sorts of programmes he might be interested in but many institutions don’t have anything like the quality of library facilities most UK universities have. Online content is also more limited at many of them (but there are ways to get around this and most departments help out). This is improving a lot but overall, access to materials in English will almost definitely be at least a bit less.
He may be one of only a few native English speakers. This isn’t necessarily a bad thing of course but it does mean he might be in classes with people who are still learning English or who speak to each other in their local language. Does he speak/want to speak other languages?
Semester/term times vary too so he might have summer holidays that don’t correspond to those of his friends back in the UK. In Germany for example, I have friends who have classes and exams until July/August then go back again in November.
As @Em says, there tends to be more flexibility with course length so in Germany, Finland, Belgium and I think Sweden you get people doing an extra year just for the sake of it and it’s affordable so why not.
I guess its also possible that student mobility might not even be an option in a few years so that’s a factor maybe.
@shrewbie has studied in the Netherlands I think
@still_here has studied in Belgium (at a French speaking uni, on a programme taught in English).