Prince Sverre Magnus has been a legal adult for over a year and has been living independently from his parents, away from the royal court, for months. In addition, the recent revelations about his older half-brother have made amply clear that the Crown Prince Couple do not exercise powerful control over the lives of their adult children.
So although the crown prince and princess, the king and queen, and the royal court's press department most likely have some input into Prince Sverre Magnus's activities and the public communication (or lack thereof) about his activities, I think it is reasonable to assume that the Prince is the one who is ultimately responsible for those choices, good or bad.
People complaining about the statement gives me, once more, the impression that there is basically no option to make it right.
They give no statement: People complain about not being told about plans.
They give a statement: People now complain that it isn't what they (for what reason) expected him to do.
The latter would depend on the nature of the currently unknown plans. Military service, enrollment in university, or any of numerous uncontroversial jobs (for example working as an IT specialist, in keeping with his choice of high school program) would surely not arouse many complaints. On the other hand, if he is planning a career as controversial as his paternal aunt's, then it would indeed be a no-right-option situation which would account for the secrecy (Princess Märtha Louise's career steps were never kept secret).
Otherwise, I find it oddly illogical that Prince Sverre Magnus seemingly believes it is more important for the public to know who his girlfriend is, and that she is close to his mother, than what job(s) he is doing or what country(s) he is traveling to.