He says "Telling you". The final "g" is, if it's there at all, so quiet it can't really be heard so effectively he says "Tellin' ya".
It's possible that there's a very slight "m" which would be part of "I'm". The sound would be made simply by using the vocal cords while parting the lips to say "telling". However, you're right that he doesn't say the full word "I'm".