He has had THREE hip replacements Does that mean he has three hips, therefore three legs? Sweet.
Unlucky for him, means one has been redone once or twice. Replacements are high tech these days but youre still replacing bone with an artificial one, each time losing more bone, and becoming a higher risk.
Make a phone call to the Specialist's office, they will quickly tell you whats o.k. otherwise book him an appointment.
He might be tough but if he's youngish and has even a simple wipe out the wrong way, (read some of these threads) youre looking at big problems... for a long time.
.. if things stuff up guess who'll be running around for him..