Parents reading IMs sent from my phone?

Ok my dad found out about this guy that I'm going out with and he hates his guts! So my dad won't let me text after 8 and he takes away my phone every night. And recently he signed up for a thing on T-Mobile that'll let him read all the text messages that are sent to and from my phone. So to talk to my bf I usually log onto Yahoo! Messanger from my phone. But I heard that IM counts as texting. Can my dad read the IMs that are sent from my phone like he can read my text messages? Please help!