I have to agree with the trend of the recent advice. While you could probably put together a bike that would be usable by both you and your son, one or both of you will be toward the limit of your ideal size range. I think a CX bike would be my last choice for such a compromise. Since you plan to ride the bike offroad occasionally, you would likely find yourself in a situation where you would be painfully aware of the lack of top tube standover clearance. with a regular road bike or touring bike, you might be able to set your son up with a modern fit while fitting yourself more in the French/Rivendell manner.