RMC your son will be fine, and should pass the vision test no problems.
There is a form you can print from the website that you can take to the opticians, get the optician to go through it and you'll get your answer as passing the eye test is more than just meeting perscription requirements.
If he want to be a pilot, then as long as he is old enough, go get the medical now. I, and i know of a few others who wish they would have got the C1 medical sooner rather than later, can make life so much easier.