I think it will come down to some fundamental choices and decisions. If your son wants to return to academia, and to pursue an academic career, a master’s degree, or possibly a PhD, will probably be needed, although this will depend on the subject, and the specific area he wants to study. If he sees this is a break from his current employment, with a return to that (or something else), different factors apply. Does he want to study at the university where he obtained his bachelor degree, or somewhere else? And how would it be funded? Has he continued to keep up with developments in his field, or will the return to study be a change from his current lifestyle?
My son has just finished the first year of a 4 year PhD, in a highly specialised area. I don’t think he has yet fully decided whether he wants an academic career, or something else, but he does have to work hard and apply himself to it. As he is developing a new theory, there is a lot of risk and ambiguity about where the research might go, and it is constantly on his mind. Perhaps not entirely what he was expecting when he embarked on this.
Edit - just read mandryka's post above. Both my children went to Imperial (and my son is still there) - and have found it to be a very rewarding place to study. I think the key point is to make sure that whoever the sponsor of the master's or PhD is, will be engaged and provide intellectual guidance, mentoring and other support. I think this depends more on the individual sponsor than the university, and also whether the research is part of a larger team, or individual.