Degifinitely agree with asking him how you can help him. And then follow that. Don't assume you can help him in better ways than he can come up with. You can ask what he can see, but only to satisfy your own curiosity, not to decide how to help him. Other than that, just treat him like any other student.

Source: my wife is blind and trains other blind people on technology

I would try to arrange a 1-on-1 meeting with him. You could explain that you'd like to be sure he's getting everything he can out of your class. Ask what format he would like materials in, and if he would like them in advance. Ask what accommodations he needs in the classroom setting and what has worked for him in the past. He's made it to the university level, so he should be able to explain what previous teachers have done that work for him and what doesn't. Every year my students ask their teachers to please remember to read everything they write on the wipe-off board. That's a common complaint they have. As far as asking about his vision, I think it's perfectly acceptable to ask. In high school I start having meetings with the student, his/her teachers, and me. They learn to state what their eye condition is, what they actually see and accommodations they benefit from in the classroom. By college, the student should be able to have that meeting on their own with you.

Ask him what format he would prefer for any materials you're going to provide. Probably electronic - but see if he wants Word, PDF, plaintext, etc.

> would it be appropriate to ask him what the extent of his blindness is

My opinion: not unless it's actually relevant.

I would find out if he is using jaws as that is the most common computer program for blind or visually impaired people, and make sure any special programs you may use or books will work with it. I have a friend I want through blind school with that is going to college now for medical billing and coding and her biggest problem is the colleges programs and other things are not compatible with jaws.