If the issue only occurs when he is driving, or otherwise away from easy access to a bathroom, the cause is unlikely to be physical. A counselor is the appropriate route, and the guess about what a counselor would tell him to do is just that - a guess.
If the anxiety stems from fear of not being able to access a bathroom, he might consider wearing an adult diaper while driving. Perhaps having a back-up plan in CASE he really had to go would help him relax.
This doesn't address the anxiety part, but there are so many remote jobs now. He should address this with his doctor, but in the meantime, he could still have a job and work from home.