Here's some possibilities:
Girth, noseband, or any other significant adjustable strap is too tight
Horse has weak back muscles
Ulcers, sore hooves or other physical issues
Rider has an incorrect position
Horses can often travel fast in a hollow or forehand heavy frame with no rider, but when the rider's weight is there they are unbalanced.
Videos would help, one of the horse being lunged and the other with the rider. The fact that you said he is coming back to work makes me suspect weak muscles, so proper lunging combined with a bit of free lunging once he is moving correctly would benefit him.
"The trouble with the world is that the stupid are certain and the intelligent are full of doubt"