I might do it and have him as a minor character, really. The issue is that he'd probably end up with Crompter* due to their similar stance. In general I see it as poor practice and try to not have two of your characters working together with one another in this type of RP, though if you don't object to it I might just make this the exception.

* Old man has a strong sense of morality and if the sandcastle story didn't show it, he hates those who abuse power. If the Prince doesn't watch his wording and actions [i]very[/i] carefully, Crompter is naturally going to be that one guy who opposes the immortality.