Hi Nori!
A second opinion is always a good idea, especially when you're considering major surgery.
I have severe osteoarthritis in both knees and am bone on bone too, so have been forced to learn about knee replacements (though I'm trying to stall as long as possible!) My doctor and I talked about the weight issue since I used to be morbidly obese and undoubtedly did a lot of damage to my knees during those years that can't be undone. (Note: I could just kick myself for being so stupid and not losing the weight sooner!!)
Anyway, yes, 300 pounds seems to be the cut-off for knee replacement from what he told me and I've read. I think the problem is that the artificial joint can't support more weight than that - it's a mechanical problem rather than a healing problem. My doctor explained to me that every extra pound that we carry exerts
four pounds of pressure on our knees, ankles, and feet and even just a few excess pounds can cause major problems. So every additional pound would also exert four pounds of force on the artifical joint and it just can't handle it.
I'm no expert by any means, so take this with a grain of salt! But I do run across obesity as a counterindication to knee replacement in most of my readings, so I don't think it's just your dad's doctor.
The advice I was given on preserving my knees for as long as possible is: 1) exercise to strengthen the muscles that support the knee and 2) maintain a normal weight. There's really lots your dad could do! There's swimming, of course, for cardio as well as stationary biking - that's very good for the knees. I can still do the elliptical but imagine that would be too much for him to start. Then there are leg strengthening exercises that he could do such as leg press, leg extensions, leg curls, squats with an exercise ball - most of these are done seated and none involve impact.
I truly believe that exercise is the only thing keeping my knees going at this point! I can actually hear the bone rubbing on the bone when I walk (gross!) but still do an hour of non-impact cardio every day and a pretty strenuous leg workout twice a week. My legs are ridiculously strong and have created a kind of internal brace for the bad knees. Honestly, ANY exercise that your father does would be helpful!
Do you live near enough that he could exercise with you? It's so much easier to start out with a buddy! Would he accept your help with a healthy diet plan? Perhaps if he lost just 10 or 20 pounds and started to feel the difference, he'd be motivated to continue.
Best of luck to the both of you.