Drewfus,

For the last 3 yrs I've gone in with my father who is in his late 60s.  He is not in the best shape either, but he is smart, tough, and knows his limitations (sort of).  Don't discount the extra toll it will take on you as the mule of the team.  After my last trip, which was my father, my brother and I, I realized that putting too much pressure on myself to accomodate the other members of the team presented as much risk as overworking or expecting too much out of the old man.  If he can carry one pack over a portage you and your group should be fine pacing yourselves.  

One thing I would be careful of is making sure he doesn't get stuck in suck mud.  That happened to my father on day one last year coming out of the Mack creek when it was a little low.   It took an awful toll on him way too early in the trip.

If he loves the woods, and knows himself then you will be fine.