He can't see well. - ok, informal. Non-clinical. I think he has a defect of eyesight. -ok, but ... I think he has a defect of vision. - ok, but formal sounding He has poor eyesight. informal. He has a vision impairment. - formal He has impaired vision. - formal/informal His vision is impaired. - also neutral - can use quialifiers, eg His vision is quite impaired. --> very His vision is somewhat impaired. ==> a little His vision is substantially impaired. - greatly NTN - No thanks necessary. Comment if you have more questions.

It is more natural to say “he has poor eyesight” or “I think he has poor vision.”

The first one is the most natural. I can understand the meaning from the other two sentences, but they are not natural. It would be more common to say "vision impairment", "bad vision", "blurry vision", "poor eyes", just to give you some examples.

"He can't see well" or "he doesn't see well" are better than "has a defect of vision." You could say "He has poor vision" also.

The first one sounds fine but is casual. A better way to say it is "I think he has a vision impairment."