From Reviews on Urban Spoon:

For years the Spring ritual in Maine is to make a pilgrimage to Fat Boy's in Brunswick. Not only is it one of the last true drive-ins left in this area but the food is excellent. This is not the place for your expensive gourmet burgers but features basic diner lunch fare.

Fat Boy's hits the spot with so many favorites such as their Clamcake sandwiches, onion rings, fried clams, and their chicken tender dinners (the coleslaw that accompanies it is the best I've ever had- so creamy).

If you've never eaten here before you'll need to prepare yourself beforehand. This isn't like Sonic's- this is a Maine institution so don't expect a chain restaurant with frozen food. Instead, you're getting fresh, cooked to order food with an original recipe.  

It's worth sampling everything on the menu. They have some AWESOME food on their menu, it's worth the drive up there and then afterward, drive on down to Bailey Island, it's one of the most beautiful, scenic drives of the coast in all of Maine.