He's probably doing this because he hasn't learned how to entertain or to self-soothe himself yet. Sounds like over the summer, you kept a pretty fast-paced schedule and he didn't have to work very hard to find someone to play with or something to do.
With that said, I think now's the time to show him how to be alone sometimes and find constructive activities to occupy himself. I'd recommend providing him some quiet activities like reading, or hobbies like trains, simple machines or sometype of craft like paper airplane making, where he will learn to be responsible for his own happiness and enjoyment. He might find he enjoys time away from others if he has something to look forward to that he enjoys doing.
Lastly, I think it's really important to tell him that as he gets older he can not expect to be entertained by others and at his whim. Explain to him that as he gets further along in school, he will need to learn how to do assignments on his own, read on his own, etc. and so will his friends. Stress there is a time for play and a time for other equally important activities.
In the end, if you start gearing him this way now, he'll be better for it. He's not too young for this either.
If you're looking for places to start looking for hobbies and interests to develop in your son, I'd recommend taking him to the library and start with some books or videos and go from there. You can also get free passes to museums at the library to some great places like the firefighter's museum, and I'm not sure if they still have them, but also Science museum passes. He might really get into astronomy and star gazing with his own telescope, or bugs with his own microscope. In St. Paul there is a train museum and many clubs for kids into train collecting and sets. There are also chess clubs..even for toddlers as young as 4 and Lego leagues too (they meet only a few times a month.. the rest of the time your kid can work on his skill at home for the next meeting where he can show off). There are also tons of age appropriate things on the market for kids his age too to help develop interests and hobbies.
One of my favorite sites is www.fatbraintoys.com. You'll be amazed at what's out there to keep active kids busy but learning along the way.
I hope this helps.
N