How do you find out whats in a text message?

I need help! With

I share a joint cell phone account with my boyfriend. I pay the bill. I saw this month that there's a gazillion text messages he's been exchanging with this one particular #. When confronted...well...he says he's been "rejecting" this girl that's interested in him. Yea BULL#. Ok so I know the times and the # of text messages that were exchanged between the two. My question is..since i'm the primary account holder..can I find out whats in those text messages? I know the federal authorities and the police use this service. If I call T-mobile I know they'll lie to me and tell me they cant do it. Anyone know anything?

Help!