Should I email my professor's colleague?
I'm a professor, and two weeks is not a particularly long time for me to get around to replying to a non-urgent e-mail.  That said, I'm not generally as good at replying to e-mails (particularly those from former students, which you now are) as you describe your professor as being.

In your situation, I would do one or both of two things:


E-mail him again.  Say directly something like "I sent you a couple of e-mails over the past few weeks, but I haven't received a reply yet.  Could you please reply and let me know that you're all right, to put my mind at ease?"  A phrasing like this is less likely to come across as demanding, and more likely to come across as simply concerned.
E-mail his administrative assistant (not another faculty member) and ask whether Prof. X has been in to the department recently, or whether he is traveling or on vacation or the like.  You can say that you haven't heard from him in a while and you're starting to get concerned.  Administrative assistants are much more likely to know whether a particular professor is in town or out of town.  A random faculty member, on the other hand, is much less likely keep track of where your professor is;  I couldn't tell you what the details of my colleagues' schedules are.