Hi Pronkster,
We own at Hillcrest and Seacastles and have been spending 4th of July in Ogunquit for many years. Nerodog had some excellent suggestions for things to do, though I might add that Hurricane in Perkins Cove has changed owners and is now called "MC". Though I have not yet been there, it is owned by the owners of Arrows Restaurant in Ogunquit, which is widely considered one of the best restaurants in New England.
There are so many things to see and do in the area. Nerodog has already made some great suggestions, but I'll rattle off some more, in no particular order

. Ogunquit has a fabulous small art museum, the Ogunquit Museum of Art, which should not be missed. I believe it is open from April or May to October. The Portland Museum of Art, should your son go to Portland, is also a great museum. In addition to the Portland Head Light, there is also the Nubble Light House in York, just a very short drive south of Ogunquit. Be sure to drive along Shore Road, not on Route 1. I find Shore Road between Ogunquit and York to be one of the most scenic drives I have ever taken. There are small boats that leave from Perkins Cove for deep sea fishing (the Bunny Clark is our favorite) as well as cocktail cruises, brunch cruises, lobster catching cruises, etc. You can rent kayaks and go ocean kayaking. Hike Mt. Agamenticus. Portsmouth, NH is also a very charming town, and has some great restaurants as well as an interesting museum called Strawberry Banke, which recreates life in the area from colonial times through the current day.
Of course, eating lobster can't be missed. There's plenty of places all around. Our favorites for lobster rolls are the Lobster Shack in Perkins Cove and Billy's Chowder House in Wells. There's also another great casual seafood restaurant in Wells called the Fisherman's Catch, and lots of wonderful restaurants in Ogunquit. There's a unique little place called Chauncey Creek in Kittery Point -- very casual, right on the water where you can bring your own drinks and order up a lobster. Kind of hard to find, but worth a stop if you're out that way.
I could go on and on! Please feel free to email me if you have any more questions. I wish your son a fabulous time!
Diane