Why does he go cross-eyed when he's tired?

When one eye turns away from an object this is called Strabismus. When someone doesn’t always have this problem it is called Intermittent Strabismus.

Lazy Eye or Ambliopia is the condition where the brain ignores the image from one of the eyes. When one eye doesn’t see well our brain may deal with it by simply ignoring it and using just the vision from the better seeing eye; the problem here is that if this occurs at an early age (before age 6) than the brain may never learn to process visual information from the ignored eye well. At any age when the brain ignores the image from one eye that eye gets weaker and weaker. Strabismus can lead to Ambliopia because the brain may decide to ignore the image from one eye so it won’t be bothered by double vision.

I had strabismus but luckily it was discovered at my 1 year checkup and corrected with surgery while I was just a baby. I still have intermittent strabismus like your boyfriend. Sometimes when I’m tired I have trouble keeping both eyes on objects. I’ve also had problems with 3D movies where I would see double instead of 3D though with the improved 3D techniques they use now I’m usually fine.

Treatment for Ambliopia is usually wearing a patch over the stronger eye to force the patient to use the weak eye. Treatment for strabismus is usually surgery to weaken the muscle that pulls the eye out of alignment though if the problem is slight glasses with some prism built in can sometimes solve the problem. If your boyfriend is only occasionally troubled by strabismus than he probably won’t need to be treated but he should mention it to his optometrist or ophthalmologist when he goes in for checkups.