I live in a suburban setting but both my sisters live in Manhattan with their husbands and three sons (ages 4-9), and they wouldn't want it any other way. They've lived there for the boys' whole lives and though I think my sister with two boys would consider eventually moving out of the city (and has mentioned it once or twice), they take full advantage of city/urban life, and love every minute of it. The boys enjoy it, too. We grew up in NYC (Queens) and while I hated it, and went away to college, and have lived in Boston now for 14 years (first in a more urban setting and now suburban with two young girls) and want to continue with the suburban life, they would never consider this life for themselves.
So I think you could make it work, and work well, but it's expensive, and I guess I would say to be willing to try it for a few years and then move back if you don't love it as much as you think you would. I think it would be great for your son, though - and having family closer, too!