Answer:
No, Langstraat cannot disaffirm his rejection for the uninsured motorist coverage. Since Langstraat is a minor, he can legally disaffirm a contract, but by doing so, he will disaffirm the entire contract, not just one or two previsions of his choice.
In this specific case, Langstraat can disaffirm the whole insurance policy contract, but he cannot disaffirm only one clause. If he chooses to disaffirm the whole policy, the result will be the same, he will not have coverage.