At Wednesday's GQ Men of the Year Party in Los Angeles, a fully-clothed Johnston told PEOPLE that he sees a little bit of himself in his son. "He is very funky," Johnston says. "He's got a lot of energy. He's always looking to mess things up, break things. He's crazy."
Johnston, 19, says that the best thing about being a dad is "just the excitement of watching him grow up. There's a lot of responsibilities you take on. I became a teenager and I grew up really fast."
Earlier this week, Johnston – who split from Sarah Palin's daughter Bristol in March – announced that he is seeking joint custody of Tripp. He claims that the Palin family is allegedly making it difficult for him to see his son.
Johnston tells PEOPLE that he is currently single, a fact that might change after his nude shots go public.
With reporting by SHAYNA ROSE ARNOLD