Yet again, I seem to have misread WNS. My interpretation of the situation as described was that you were making 20kts towards the harbour. As you approach the fairway buoy, your son nips down to get the fenders out of the lazarette. Maybe he left his wife and kids up on the flybridge, maybe not (it's irrelevant). If dad told him not to go because it was too rough, he'd probably think the old man was going round the bend, given that he's been messing about in boats for the last thirty odd years.

A few seconds later, you pass the Fairway buoy, and slow down to the 6kt speed limit. A few minutes later (5 minutes @6kt = half a mile) you are right in the entrance, about where you would expect your son to be appearing with the fenders... but he is nowhere to be seen.

Maybe he was having a pee off the back when you slowed down, and the change of motion made him lose his footing. (It seems a more reasonable explanation, to me, than the prevailing assumption that he opened the transom gate as some devious route up onto the side decks!)

Having decided that he has probably gone OB, there is no point messing about putting waypoints at a spot where the MoB isn't: Job 1a is a Williamson turn to get back onto reciprocal course.
Job 1b (ideally at the same time) is a Mayday.

Resisting the temptation to accelerate, keep all eyes peeled for about three minutes to get back to the casualty's most probable position.

There are umpteen search options, but in this case we need something simple: I reckon an elongated expanding box would be favourite. No need to mess about "allowing for tide", because you and the MoB are on the same bit of water, you and he have both drifted the same distance and the same direction. Wind might be a problem, though: you will be sliding downwind faster than him, so he will probably be somewhere upwind of you.

The biggest, single problem will be avoiding panic -- not only for your own sake and your son's, but especially for the sake of your wife, daughter in law, and grandchildren.