Hi Robin,
I'm afraid there is no easy way out of this one. Noone likes confrontation, so we tend to ignore things until they are really out of control and then we blow.
Your feelings are normal. You love him but you want your privacy. Men in general are not very good on picking up "vibes" and they probably will never understand your need for privacy, so you are just going to have to politely spell it out for him.
You said it so nicely here, now say it to him. Let him know you appreciate how helpful he is (lucky you!) and you love the bond he has with your son, but you would appreciate it if he called before coming over and not to call earlier than 9:00, 10:00 (whatever you deem appropriate) When he calls let him know "I will be stepping out at such and such time, etc." and then give advanced notice "OK dad, I have to go in 15 minutes", whatever, and that you need to get ready"
It might also be helpful to try and direct his attention elsewhere. He obviously is still very healthy and active. (Is he a widower or divorced?) Perhaps there is some social network locally for seniors which you could encourage him to get involved with. If he has a particular interest, (gardening or tinkering with cars) there may be some club in the area he could participate in. You need to find a way for him to focus all of his positive energy which still allows you the freedom to joining a moms group or pursue some of the lifestyle which new moms need and enjoy.
Best wishes, all I can say is you do not have to feel guilty about making your own needs known. If you do it politely, and reasonably than it is not your responsibility if your father in law is offended. You can only be responsible for yourself not someone elses reaction to a totally reasonble request. Be strong
J. L.