When you are as young as your son they generally want you to wait as long as possible to replace them.
At his age it’s for sure he would need to replace the knee one more time in his lifetime. Maybe two other replacements.
I’m 60, almost 61 and they are wanting me to wait a couple more years to replace my hips so that we reduce the chances of needing a second or if I was younger maybe a third replacement.
Even though these surgeries are generally pretty safe, any time you violate the human body like that it’s never good.
The less times they open you up the better off you are. Just the risk of infection is enough to not want to have a replaced knee or hip more than once.