I have a 12 year old nephew, and this problem has come up before. I know it breaks your heart, but kids are very grade-centric right up to high school. It's only a two year gap, but to kids that age, it might as well seem like a million years. To an 11 year old a 9 year old is a "baby" compared to them. Interests can vary hugely in just two years, and even though your boys are brothers, and your younger one is well behaved, the older one is attempting to assert his independence as an autonomous person separate from his brother– with separate friends and separate activities and separate memories. They'll always be brothers, but trying to make them do everything together and be included in everything together as a unit or is just going to cause more problems down the road.
The best thing you can do for your boys is to make sure they have some quality brother-time, just the two of them away from the other neighborhood kids, so they can have those good memories— and other kids and any unfairness don't enter into that equation.
Respect your older son's need to have separate friends and activities and explain that to your younger son. It will be harder for your younger son to eventually become as independent as he needs to be if he is always tagging along with his older brother and "copying" him by doing everything he does. And if you force your older son to always be a package deal with your younger son, he's just going to begin to resent you and it will form a bad dynamic with his brother.
Encourage your younger son to get involved in some activities with kids in his own grade and age where he can make friends. See if there are YMCA day camps, or free workshops, or museum days, or neighborhood activities that he can get involved in this summer. See if he'd like to start a hobby, that's a great way to make friends with the same interests.