Don't be overwhelmed with the idea of "tests" as it likely means stool samples (easy with diapers), and blood work (a one time blood draw, just a bit more than they would normally take). If he's as high energy as you say, there's probably nothing wrong, but if your son does have an absorption disorder, you'll feel better if you do something about it now (before it hinders his lifelong growth). And if your son is fine, the Dr. will shut up about his weight except maybe to suggest you improve his calorie intake.
While I wouldn't suggest eating at McDonalds, I would work on replacing existing choices with a variety of more calorie-dense foods: e.g., whole milk with 1T half-n-half per cup, light cream on cereal, give waffles with butter and syrup, french toast instead of regular toast, add extra cheese to mac-n-cheese, use a higher-fat content cheese like havarti, provide peanut butter, nutella, or cream cheese for crackers or veggies, and use higher fat% hamburger, etc.
If your son is old enough to chew well, you can give nuts (or make pb cookies, cinnamon rolls or brownies with nuts) as special dessert treats.
My daughter was only 24 lbs when she started Kindergarten, and didn't really grow more than a few pounds a year until puberty. She is now a normal height and weight (about 18% body fat at 5'4").