[Iluvbertie]

You are assuming that the Commonwealth want him.

I have read a number of suggestions that when the Queen dies Charles won't automatically become the Head of the Commonwealth as many of the countries feel that the Head should be a rotating headship. In other words there may be less of a role for the royals in the Commonwealth in the future.

In addition what would they do?

The Queen spends her holidays at Balmoral and Sandringham. Imagine if they were holidaying for months at a time in foreign countries.

It isn't as if any of the countries of the Commonwealth want them to do any official stuff so they would be doing not much at all.