You should stop your son to do such behavior. Or else, he may get the crossed eyes which is a kind of eye disease. Once he gets the crossed eyes, he may use the special eyeglasses to adjust the eye sight. You should not let him do such thing often. You should tell him the dangers of doing so. The eyes are the important parts for the whole body. We should better protect them carefully.
Well, generally speaking, from your description, I have to say your child may suffer from crossing eyes, which is also named as strabismus. Strabismus refers to the condition in which eyes don't look at the same point at the same time. And according to some experts, some exercises can just correct it. So you can just have a try. First, you can place one of your index fingers on the tip of your nose. And then, keep both eyes focused on your fingertip, slowly moving your finger away from your nose until your arm is completely straight. Finally, what you should do is to slowly return your finger to your nose, staying focused on your fingertip the whole time. And then, just repeat it.
Don't worry about it, cross-eyed will not affect your son's eyes or vision. Many people try to learn how to crossing there eyes for fun. To crossing your eyes, you need work your near vision. It gives a special feel when crossing your eyes and some people even say that it can help practice your eye muscles.
However, things will develop in the opposite direction when they become extreme. If you constantly cross your eyes, your eyes may get use to the condition and sometimes you may cross your eyes involuntary. But it happens rarely. Tell you son that eye is one of the most important organ of human body, so donâ€™t mess with your eye.