I totally agree -
1. The decision is his. Why are we so darn protective of older folks, as if they had no sense? You gotta LIVE!!
2. I am near 68, wife near 70 - I plan on riding into my 80's and 90's if I make it.
3. If his balance IS bad (I really doubt it) then there are always excellent, lightweight trikes.
4. Start easy, ride with him. You will both get more fit.
And, yes I am a super-fit athletic, trim, stud! (Don't I wish!)