I don't think it is reasonable to hold one child back because the other can't do a thing. If one son was asthmatic and couldn't play certain sports, would you say neither should be able to?
That's about the perfect age to start adjusting to contacts, IMO, so I wouldn't say no to the child who can wear them. It is easier to get used to putting them in/taking them out when young, I think, and disposables have made the care and upkeep much easier. I've been wearing contacts daily since 11 - my eye doctor usually held to 13 as a minimum age, back before disposables, but made an exception for me because my bad eyes were getting in the way of my sport - and they've gotten so much more user friendly over the last 30-odd years. Also, so much less of a commitment for a kid wanting to try them because you can buy a single box for each eye to see if your son really will wear them and take appropriate care of them before ordering a full year's supply.