Did the doctor mention osetoarthritis or rheumatoid? Just wondering how he "knows" your son will need knee replacement.

In addition to the testing Leesa cited, the doc might also want to do a bone scan. I know that with my onset (age 22 when it was terribly bad), no doctor had done the testing that could have "displayed" evidence of arthritis. A bone scan woulda done it, only... missed opportunity.