Can't think of too many things better than riding Busas with my son (or father many years back). You and your Dad are lucky men.
Take it from a 50 year old guy who's Dad is no longer around, make sure you take pictures when you ride together and maybe even some video. They will be priceless some day. Here's an idea...Take a picture of you and your Dad with your bikes the 1st time you ride together. After posting them here, frame the picture and give it to him on Christmas or his next birthday. He'll look at it every day and show everyone that comes into his house.
Alright...need to stop. God I miss my old man.