Be firm and honest with him, but be respectful. My MIL was very similar...always meddling and trying to assert control over things that were not really her business. I asked her if we could have a private conversation, put it all on the table in a kind and respectful way...but still drew a very clear line in the sand. Things have been much better since. Also, it is important that you and your wife present as a united front. If your wife does not support your "message," nor will your FIL.