My boyfriend was recently put in the hole. How do I find out exactly what for? I’m concerned he may be lying.

Inmates can be placed in the SHU for a variety of reasons. there are two designations, one Administrative and the other Disciplinary. If he was put in Administrative Segregation, it could be for their own protection if there is a credible threat. The other is if the rules were broken. He might have been in a situation where he didn't do anything but gets locked up for "investigation", and won't get released into the general population until there is a final disposition of the incident.