What is too much on a bicycle for an older man?

I'm basically asking this for my older brother. Many may know him here as "Old Hippie". He'll read this sooner or later & get mad! Remember - he's pushing 60 with arthritis in lower back & knees. We went out riding early on Memorial Day & did almost 15 miles. He took his Mountain Bike which was a surprise to me. Afterwords, he said his lower back seemed OK - which is also another surprise. But he keeps complaining he should be able to do more. Two years ago he couldn't do 5 miles. On his Hybrid bike, I've seen him do 25-30 miles. I think he's doing fairly well considering he's lost about 60 lbs. Sure beats watching TV all day. Given his physical ailments, do you think he's pushing too hard? Just one brother worried about another...