I don't believe lactose intolerance can cause weak teeth unless the kid is not taking any other sources of calcium. You may want to test for calcium deficiency. There are plenty of ways to add calcium to diets. Please check with a pediatrician in that regard.
Fariborz Rodef, DDS
Calcium-fortified soy, almond and rice beverages
Calcium-fortified orange juice
Vegetables such as bok choy, broccoli, rapini and kale
Canned fish with bones such as salmon, sardines and anchovies
Soybean-based foods such as edamame, soy nuts and tofu
Beans like chickpeas, kidney beans, white, navy and pinto beans
Nuts and seeds like almonds and sesame seed
Permanent teeth start to develop slightly before birth and afterwards, so a lot is dependent on prenatal and early feeding habits. The strength of the teeth is more genetic in nature, but prevention will help protect the teeth. The importance of calcium in good bone development (including the jaw bones) is very important, but remember, cows milk is not the only (or best source) for calcium, after all, your son is not a calf, but a human (I am not a big fan of cow's milk). Great sources of calcium include broccoli, kale (and other dark green veggies), beans, and fish. Also, some OJ and non-dairy milks are calcium fortified!
Eat healthy, brush after meals, and smile, smile, smile!!