I love free-range kids, but I also think it depends on the neighborhood.
Would he be walking or on a bike? Are there other kids to hang out in the neighbhorhood, do you know them? Do you know other people in the neighborhood well? Do you have their numbers, etc.?
When I was young under 5, all the kids in the neighborhood would all hang out together almost all day long with almost no adult supervision, but it was all ages of kids and the older kids watched out for the younger kids although at the age of 3 and 4, I knew not to cross the street, go with strangers or go anywhere I didn't know and I didn't go anywhere alone really except maybe out the back door to the playground or I'd walk the sidewalk directly in front of my house. With the group of us though we'd walk the whole block and further.
I live in an apartment complex too but I don't feel comfortable letting my 5 year old go out by herself. Maybe if I sat on the patio and there were other kids out for her to play with I would.
At the last place we lived I would let her outside to play in our lot(we lived in an RV park) or to go to the neighbors 1,2, and 3 doors down, but it was a very friendly, small neighborhood everyone knew each other, there was almost always someone outside and if she was out there awhile the other kids would certainly come out to play with her at some point. I didn't leave her out by herself for very long though, she was 3.5-4 at the time. There was also a mutual understanding that people would watch out for all the kids and we could see and hear her from inside, we often kept the door open too if the weather was nice, without watching her. She knew not to go anywhere on her own, obviously not with strangers and she didn't cross the street.
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