The 4th leg is a "spare".
Within 3 weeks of having his front right leg amputated due to Osteo, Taylor was jumping in and out of the back of my SUV.
He was about the same age as your boy, and a BIG boy (90 pounds).
He had 4 once a month rounds of Carboplatin chemo, with no side effects.
He loved and demanded rides and walks, 2-3 times a day. Every single day, for almost 2 years, until something else sent him to the Bridge.
He was a happy, hopping tripod.