At seven, he's in school. Is he also in any activities such as Cub Scouts, or a church or synagogue kids' group, or a sport or martial art, or anything else?
I ask because all those places are where he meets other kids and it's time for you to start having play dates with kids from school and his other activities. At his age you need to be the one to initiate things and set them up and drive him places if that's part of the equation.
It is so easy to just send him down to knock on a neighbor's door, but take it from a parent of an older kid: The whole "hang out with neighborhood kids" thing is fine but really trails off as children get older and get more involved in their school and other activities. First, there isn't as much hangout time. And second -- we all, kids and adults, tend to form our real friendships based on shared interests and shared activities, not merely on how many feet our house is from someone else's house. Yes, he'll have neighborhood friends, but he also needs friends with whom he has some shared interests and experiences etc.
If you are not already involved as a volunteer at his school and/or with his other activities (every activity needs volunteer parents!), get involved now. It lets you get to know other parents and see your kid relating to other kids, and makes it easier to say: "Hey, Johnny, I notice you and Jimmy were really sparring well together at tae kwon do! Do you want to have him come over and play this weekend?" Or, "You hang out at church youth group with Billy sometimes, right? Billy's mom said he's really into Legos, just like you are. I talked to her and we'd like to get you guys together at the Lego store at the mall for a 'build' they're having this Saturday. What do you think?" and so on.
You won't have to manage his social life forever but yes, for a time to come you will. It's too bad if his best buddy on the street hasn't been available to play but make other play dates. (Please never, ever tell your son, "We're going to do more play dates with other kids because Neighbor Friend isn't paying much attention these days." Never....Just make play dates, always asking your son about them, and don't bring the neighbor kid into the discussion.)
And do give the neighbor friend and his family the benefit of the doubt! There may be things going on about which you have NO idea -- illness in the family; trouble between the parents; maybe the boy's having issues at school; there can be a lot of things that make a kid or a family have to lie low for a while, and that may have absolutely nothing to do with your own son or how much the neighbor friend likes him. And truly, neighbor boy may indeed simply be busy. Approach it as "Maybe they're having a tough time I can't see" and give them some breathing space. If you know the mom well enough, rather than saying "Why won't YourSon play with MySon?!" just ask, "Hey, how are things with all of you?" and leave it at that.