Fetlocks are a very unforgiving joint. Avoid overly hard footing, but don’t go too soft either. I like to poultice with Sore No More clay after I ride, and rub with Sore No More (or plain Arnica gel) prior to my ride.
Previcox is an every day thing, not an “after jumping” thing like bute. It doesn’t work that way.
The best thing for arthritis is movement, don’t let him get overweight, good solid farrier work… you could try Adequan or maybe even Osphos (??). I used Legend before but never saw any sort of results.
Do you know why his fetlocks are giving him trouble? Does he have chips? First thing I’d want to know why, then I’d look into therapies.