Our eye doctor said to not let my daughter wear her glasses while doing sports when she was that age. He said as long as she didn't need them to see close up, it was better not to wear them. So she took them off and left them with me or a responsible adult. At first she only used them when doing school, reading or watching television. Eventually she did need to wear them all the time, but by then she had gotten used to taking care of them.
Be sure to get the most flexible sturdy frames available. Kid's lenses are made pretty well to be light weight and break resistant. Also I would recommend a place that has a replacement warranty, even if you have to pay some to get it. With a kid that young, it will really pay off to have a warranty. Even if he is very careful, something will happen to his glasses. We replaced my dd's first pair four times.
Another daughter had her glasses chewed by the dog three times. The first time is was her fault, but the second time, the dog really went out of her way to get to the glasses. Must have acquired a taste for them!