I have two sons. My older son has a friend whom he brushes off but yet my 11 year old son really likes. This friend is a good kid. My older son is upset that his friend is playing with his younger brother. My younger son really enjoys this friends company and they get along great. Is this okay?
I don't think your older son really has a choice in being okay with it or not. If the friend and the younger child want to be friends, hang out, and do stuff together, your son can either tag along and be cool with it or find another friend. Your older son is not in possession of this friend. He's not a toy that he can refuse to let his younger brother borrow. Ultimately it's not up to either your older son or younger son, it's up to the friend.
I went through the same thing with one of my siblings in dealing with a friend. I was in the same position as your older son. Initially, I pouted and made myself feel miserable while they were out and having a grand time. Eventually I realized that the only person who was mad was me. Finally I chose to hang out with them when they let me and all three of us had some pretty awesome times.