Talk him around to a none work day trip, do a test trip using the route, I do that on my road bike on weekends when I find a new route or something is new that changes my riding to see how long it takes me to get there and how I feel doing it. For someone who needs their fitness up at a high level I have shocked myself with some routes I thought myself to be "No problem, I've done longer trips." Time the trip and if you both end up being "late for work" then it is only a mock trail. As mentioned before by others, the more confident/experience he gets the faster he might go for his age.
Good luck, I hope he does end up riding with ya without problems.
