He is WAY up the Potomac so all he has, is a skinny, winding channel flanked by very shallow waters. I'm not sure how much commercial traffic he'll be dodging, so I figured swing keel/centerboard/shoal draft was really important to keep him from getting stuck when he dodges outside the channel.
Also, very light breezes up there, so something light that doesn't take a lot of breeze to get moving.
I kept a powerboat with a sailing dinghy on the Occoquan for a couple of years. I spent a winter at the Gangplank marina on Auspicious. When I was young I sailed a Cape Dory Typhoon out of Washington Sailing Marina.
I agree that shoal draft is good for those waters. I'm not sure what your concern is about the heavy keel of boats like the Catalina 22. You might want to explore your concerns - maintenance of the pin, cable, and winch is easy and infrequent. Regardless, you may have your reasons and you should be comfortable with your choice. I would encourage you to consider the Catalina 22 in addition to the Morgan 22 and similar boats. I do strongly advise you to avoid wing keels. Wing keels become great anchors in Potomac mud.
Either of those two boats (and many others) will provide great sailing along the upper and middle Potomac, with wonderful opportunities for adventure as far South as Colonial Beach and even Coles Point.
I sailed a Catalina 22 on the Bay for a while (as well as J/80s, J/22s, and assorted other small boats) and you'll have a great time even if you decide on an extended adventure down the River and into the Bay.
My suggestions are along the lines of Bubblehead. Do look at the Catalina 22 and 25. You have to haul ANY centerboard or swing keel boat before purchase to check for growth in the trunk, which can be a real job to get out. You'll probably have a port-a-potti - check the rubber and plastic bits for decomposition. Everything else is pretty straightforward and right out of the "so you want to buy a boat" books.