R.M.
M.,
A friend of mine had the same thing happen when her oldest started school. She explained to the older child that his little brother naps during the day and is not ready for bed quite as early as he is. Plus, going to school and learning so much can make a kid more tired than if he was just playing at home. You could always reason with my friend's older child, he saw the logic and accepted the situation.
It might not work for all children, but it might be worth a try. Then if you wanted to slowly change the younger son's bed time you could go to bed 15 minutes earlier every few days. I would encourage any child to nap or at least rest for as long as possible.
Good luck,
R.