Sibling Rivalry When There's an Age Gap
That will be a cold comfort to the big brother who doesn't want his "shadow" constantly following him, though. In this case, you need to realistically assess the situation and determine what's really happening between the two kids.
First, consider what role you are playing in this scenario. Do you encourage your older child to take his little brother along because you're trying to avoid a 6-year-old meltdown? Are you pushing joint outings because they are, in effect, free babysitting? It's reasonable to insist that your kids spend some time together, but make sure that you aren't putting unfair pressures and burdens on your older son.
You also need to make sure that your younger child is developing his own set of friends and interests. If he's spending too much time trying to be a faux-preteen, he may be missing out on age-appropriate peers and activities. Encourage the younger boy to invite friends for a playdate. Look for activities (Little League, Cub Scouts) where he will be surrounded by kids his own age.
If the two kids aren't forced to spend all their time together, it will make the moments when they are in each other's company more appealing.