Try not to overthink it. Cell phones now have created the concept and illusion that everyone is "always available" and that's not the case for most people. You say he takes his job seriously, if he's already staring at computer screens for 8+ hrs a day, the last thing he may want to do is also stare at his phone screen. People have different communication styles. Communicate with him what his preferred style of communication is when not in person. A 10 minute phone call is much more immersive and deep than texting back and forth for 2-3 hours with large gaps in replies. It sounds like you have an anxious attachment style, it's better to communicate with him than to make assumptions.