On the lying question, the honest answer is that incarceration does not create liars. It reveals them. Someone who lies to a faithful partner from inside a cell is someone who would lie on the outside too. The circumstances changed. The character did not.
That said, not every inmate is running a deception operation. Some people are genuinely doing their time, staying focused, and holding onto the relationship they have on the outside because it matters to them. Those inmates exist and they are not rare. The mistake is assuming that because some inmates behave badly, all of them do.
On pen pals specifically, yes, some inmates have outside correspondence with people they did not know before incarceration. Pen pal programs are legitimate and widely used. For someone who has limited outside contact, a pen pal can provide mental stimulation, connection, and something to look forward to in the mail. Whether that crosses a line depends entirely on what the correspondence involves and whether it is being hidden from you.
The question worth asking is not whether pen pals exist in the abstract. It is whether your specific person is being transparent with you. An inmate who has a pen pal and mentions it casually is different from one who is hiding correspondence and lying about it. The hiding is the problem, not the letter writing.
If your gut is telling you something is off, trust it. Not because all inmates lie, but because you know this particular person and something in how he is behaving has registered as wrong. That instinct is worth paying attention to.
If he is lying while you are being faithful and waiting, that is information about whether this particular vehicle is worth riding out.