My son (now 9) did the same thing around that age. We took him to the eye doctor, had a bunch of tests done, and were eventually told there was nothing wrong with him. He would blink alot and his eyes seemed to be just the slightest bit crossed. I, too, attributed it to him being tired or eyestrain from the TV. But the doctor said he was fine. It took awhile, but he grew out of it.
Still, you know your child better than anyone. If you feel he is in distress, or if the problem seems to be getting worse, by all means get him checked out by a doctor. Sometimes a call to the pediatrician can put your mind at ease as they can tell if you need to be seen right away. Give them a call in the morning just to be safe. :-)
Best of luck to you!