Best Friend Issue

I am sort of in a sticky situation with my very best friend, which we'll call Bob. Bob is actually a male and is homosexual (which I have nothing against or anything). Bob is also slender at a normal weight and tall (6' +) and is a smoker. Lately, I have noticed Bob has lost MAJOR weight (20 or more pounds) in a very short period of time (like 2 months) that he didn't need to lose. I mean 5-10 pounds lost would be a lot for him to lose because he is lanky and skinny. His teeth have started to brown and yellow. Normally, his teeth are pristene (sp?) and would normally be horrified at this turn of events surrounding his teeth.

Granted, I know STDs, HIV, and AIDS can affect anyone no matter the race, gender, or sexuality. I also know Bulimia and Anorexia is a growing epidemic in the homosexual community . . . it's now affecting a lot of gay men, which he is. He could also have cancer . . . who knows?

My issue is this: I am concerned about his health because of the weight loss, teeth decolorization, and the fact he just looks sick. I don't know how to confront him and help him to get to the doctor for treatment for whatever he might have. I have thought about just asking if he was feeling all right . . . if he'd been sick lately. I don't know if I should point out he's lost a lot of weight or not. I wouldn't mention the teeth situation until the whole thing came out and there was an appropriate time to do so. It just worries me that his weight loss might be something very serious which he might not be getting treatment for. Any advice?