The guy I like is online, should I IM him?

We talked in school today. He's in my Homeroom and History cles. He gave me his number when I was going through a rough time and said I could always call him. He's always sweet and funny with me. Right now he's grounded and I'm home alone. He's on, but I'm nervous for some reason to contact him even though I want to. Should I? What to say?