i'm going to point out that at this point though it isn't really an "intimate marital issue" anymore... if OP is able to readily observe all of these things about the father's behavior change and suspect what's going on (the texts are pretty strange), his father's lack of discretion has turned the marital issue (if there is one) into more of a family issue. if OP doesn't do anything it could end up weighing on OP and affect his life adversely, and so then it becomes his problem.

you are right, children are not privy to the details of their parents' intimate lives and shouldn't get involved in their parents' marital issues, but the more IMPORTANT meaning of that statement is that parents (as the elders/adults in the situation) should be responsible enough to be discreet if they have some kind of arrangement they don't want their kids to know about, and also to just not cheat (since that affects the entire family).

that said, what to actually do about it depends on how long it's been going on. if it hasn't been that long, maybe OP's mom will find out on her own soon, maybe it's really just that his father is working a lot, maybe the texts are just texts and nothing else... maybe, maybe, maybe.

i think the main takeaway here, OP, is that you should decide when your father's behavior is too much of a distraction to you and starts affecting your life (school, work, whatever you do) adversely. if that is now, then you should decide how long it needs to go on or before you feel the need to say something. if i were you i would wait to say anything until you could post here saying "i know my dad is cheating on my mom and i don't know what to do" otherwise, you're just going to get a bunch of advice from people speculating about your family's life and how to best go about things when really none of us know you or yours personally (and this is a very personal decision).

if you do want to say SOMETHING to help clear your head, might i just suggest asking your mom if everything is okay. you could just say that you noticed your dad has been acting differently and you were wondering if everything is okay. be tactful in how you respond though if she asks what you mean by acting differently -- like "oh just been on the phone a lot, isn't around as much..." if she knows what's going on, she'll get it. if she doesn't, she'll think about it, and if something is going on and eventually it comes out, your father can't fault you for anything because you didn't say anything accusatory or suggestive to your mom -- you were just concerned about his change in behavior.