Too much is where you start to cause problems with healing. There are multiple layers of sutures, and the inner ones take months to heal and dissolve, so I wouldn't even guess where that line is.
My routine was to get out at least three times a day, and try to be out for at least thirty minutes each time. That was about the limit to fill a pad once the catheter was out. I still try to walk at least an hour each day, but better two. The distance is not as relevant as you might think. The first weeks it took me 30 minutes to get to my complex mailbox (about a quarter of a mile round trip, and I was walking with a cane). I split up my kegels and did one third after each walk.
Just get him into a habit of some shorter walks in an area where he can get back home easily if there were to be a problem.