Rod, here are three places. One is a pier and the other two will require you to rent a boat at each location.
The pier at Matapeake State Park; first exit after you cross the Chesapeake Bay Bridge. Pier crabbin' can be OK, but with the hokey season we've had thus far this year it will be a hit or miss proposition. Map is at this URL:http://mddnr.chesapeakebay.net/fish/mapquery.cfm?Facility=QA003
Sandy Point State Park rents boats with low HP motors. You want to exit the pond where the boat rental is and enter the Chesapeake Bay. As you exit the pond both bridges will be on your right. You'll also see many crabpots. As soon as you can find an area with only a few crabpots, cross under both bridges and continue south along the western shore of the bay for a couple hundred yards. You'll see some coves where you can anchor. Crabtraps and hand lines work here. Move around until you find the crabs. Map is at this URL:http://mddnr.chesapeakebay.net/fish/mapquery.cfm?Facility=AA001
Now, about 17 miles down Route 50 is a right-hand turnoff to Route 213. Follow this to Route 662 and Wye Landing Lane. At the end of the lane is Wye Landing and Schnaitman's, where you can rent a rowboat. Jumbo crabs here. Arrive early as the rentals go quickly, especially on weekends. With rowing, you won't want to go too far away from the docks. The cove, immediately across from the docks has crabs, as does the shore line to the right of the docks. Now, maybe you know someone who will loan you an electric motor/battery combo, so you don't have to row. If you and your son really get into crabbin', then you can pick up a low HP used outboard to put on Schnaitman's boats for a couple hundred bucks. Handlines and crabtraps work here. Map is at this URL:http://mddnr.chesapeakebay.net/fish/mapquery.cfm?Facility=TA001
Good luck and let us know how you do.