If you and your Dad have different practice styles, you could suggest that you sometimes take the lead in the practice sessions or that you alternate between who leads. (I don't know if there is a significant difference between your playing abilities / levels and this will have a big impact on what is appropriate. Equally I don't know how often you and your Dad practice together, but I imagine that it is pretty regularly.
Also go with some solo practice and the duets approach as already suggested.
Talk to him, he's your Dad, he'll get it, even if he looks slightly crest fallen at first.