When things like that got really engrained in DS, I stopped buying the food - ie/ no more bananas in the house for a week or so. Then they are on rotation - I will buy "bananas" one week and not the next.
In terms of how much is too much banana, one counts as one fruit serving a day, so I would not let him have more than one medium sized one. I also try to buy the ripest possible and then let them sit for a few days until the skin has thinned and is brownish - as the less ripe they are, the more they cause constipation and the more filling they seem to be. But I personally don't see any harm in one banana every day.