It is more about the learning and skills he would have got in his chosen unit.
If he arrives at his unit with a good attitude and is keen and motivated, then opportunities will present themselves, courses, attachments, eventually even a transfer to the desired unit is possible (but depending on the specific units there may be a lot of moving parts there that need to align, so set expectations accordingly).
Without wanting to sound patronising, when you're 18, 4 years feels like forever to be stuck doing something you didn't want, but you get out what you put in and the Army is usually enthusiastic about people wanting to "better themselves" for want of a better term.
I'll say this as well, very few people really know before actually arriving at it what their "dream unit" is really like, and what their job will actually be like (even within a single capbadge there is a wide variety of jobs that need doing). He will find something that he can get his teeth into - if he wants it. And then doors will start to open.