I have a very tight male client in his early 60's who starts experiencing bad nausea during class. He initially complained about it when performing any rolling exercises, but now has it about 20 minutes into the class. I need guidance as to why this may be happening, and solutions to dealing with it. Thanks so much.

Possibly a vestibular problem? he could ask the doctor or physio to do a movement test.
I had a client who felt nauseous whilst lying down, she has managed to resolve it now as she swears it was acid reflux, she's stopped drinking certain drinks and not eating before coming to class. I'm not sure if it was definitely that but it seemed to work for her. Low/high Blood pressure is an option too maybe?

My husband has something similar when he practices Pilates. He can't be on his back for too long, because he's a giant dude (6'4 and broad) and his blood pressure doesn't regulate well from that position. In short, the instructor sits him up more frequently now and he's fine, but if your client isn't sure, he should probably check with his doc.