Trust your instincts. They are usually pointing at something real.
When someone who was calling and writing regularly goes quiet without explanation, that change means something. It does not always mean what you fear most, but it always means something. Maybe he is dealing with something inside he has not told you about. Maybe the relationship has shifted on his end. Maybe your instincts about the ex are accurate. Whatever the reason, the silence itself is information.
Here is what experience on both sides of this situation teaches. Inmates who are genuinely committed to someone on the outside find ways to stay in contact. The phones are available, the mail works, and people who want to reach you do. Consistent silence from someone who used to be consistent is not a logistics problem. It is a priority problem.
The harder truth is that waiting indefinitely for someone who is not making the effort to stay connected is a bet on a future that does not have much evidence behind it right now. How someone behaves during incarceration, when they have very little else to do and communication is one of the few things within their control, tells you a great deal about where their head is.
You have a life on the outside that deserves your attention and investment. Pouring energy into tracking down reasons for the silence or trying to compete with whoever else may be in the picture is energy that belongs to you.
Give it a defined window. If the communication does not return with a real explanation, let that answer be the answer.