Need help or advice !!

6 yr old dog that had been on that many hogs should be solid as an oak tree on what his mindset should be.

Agree on the invisible injury.

Something is ailing him to have a complete change like that especially at that age. Figuring he has been cut down before and that isn't his first rodeo.

Inflammation in the joints, ongoing infection are just the start of a list of things it could be without visible signs.

Other thing to consider.........i figure that's the first strenuous activity he has done since the wrecking? Full strength and endurance has to be built back up........if in fact he is completely healed with no permanent damage.

Watch your dogs movement and mental status ......they will tell you all you need to know if you take the time to observe.