S.B.
I would make an appt with a pediatric opthamologist (not optometrist) right away. The fact he is covering up his bad eye himself means it is significantly interfering with his vision but the good news is that it is usually treatable. The dr. is likely to prescribe a patch which goes over his good eye, to force him to use the muscles on the weaker eye. My daughter started being patched just before she turned 4. Several years of this was not successful, unfortunately, and she did have surgery on that eye when she was 10 - huge improvement, no more sliding eye, no more double vision. Good luck.