My sons dad is in prison

My sons dad is in prison and my son doesn’t really know him. Never has and he’s almost 4. But his sister messaged me and asked if we could write him and I’m considering doing it. And as stupid as this makes me sound how do I start the letter? What do I say to him? I’m going to tell him how our son is doing but what do I start off saying? I’ve never wrote anyone in prison 🤦🏻‍♀️