The answer is a stern cough while cleaning your guns.
Then eye him carefully while sharpening a rather large knife.
Make lots of "deep in thought" sounds while doing the above.
Then invite him out back to help you dig a hole.
Should work wonders for his enthusiasm.
When said son-in-law finally did ask, my husband made both of them write a 3 page essay explaining why they thought they were ready for marriage.
They've been married almost seven years now, so I guess they were actually ready.