Yes, it is worth paying attention to, but it is also worth understanding what is likely driving it before drawing conclusions.
Distance in letters from someone who is incarcerated is extremely common, particularly in the early months of a sentence. He is sitting with a lot. Depression, shame, anxiety about what is happening on the outside, and an imagination that tends toward the worst possible scenarios are all standard features of the early incarceration experience. The distance you are feeling in his letters is more likely a reflection of what is happening inside his head than anything about how he feels about you.
The questions are worth backing off on for now. When someone is already overwhelmed and struggling to process their situation, a letter full of unanswered questions can feel like pressure they do not have the capacity to meet. That makes the distance worse, not better. Give him room to write what he can without the weight of a list of things he has not addressed.
What helps more than questions is sending things that give him something to focus on. Books he would enjoy, a magazine subscription tailored to his interests, puzzles, anything that fills the dead time and gives his mind somewhere useful to go.
The harder truth is that if there are things between you that remain unresolved, questions about the past or about what led to where things are now, those are not going to get answered through letters while he is inside. Moving forward requires making a deliberate choice about whether you can set those things down and focus on what comes next. That is a significant decision and only you can make it.
If you want it to work, give him grace right now. He is hurting and he needs patience more than he needs answers.