Soarin’ is the only attraction at WDW where guests are sitting with their legs dangling.
All other attractions have ‘floors’ .
I have never heard anything regarding guests with prosthetic limbs having issues with Soarin’ . If you are concerned, you can talk to the CM at the attraction entrance. Worst case scenario in case you are concerned about the limb not staying on securely would be removing it before riding.
If you go to AK, Flights of Passage has restraints that hold the legs in a particular position. Since his is a BKA, he might not have any issues. There is a trial ride car in front of the attraction where he could try it before going inside. There is also a thread about the Pandora attractions stuck near the top of this board. It includes pictures and videos that might be helpful.