I've actually been doing some research on this option for my daughter (2nd grade now). I know where we are (in PA) there are a lot of different options for the online schooling. Some require more specific times for logging on, being part of online discussions or lessons, and are basically more strict with having to do certain work at certain times. Others are more flexible, and while they do provide a curriculum and certain things must be accomplished at each level, you are able to set your own pace much more and can do most of it as it fits your schedule. For both types, some of the work is online, but there is also work done away from the computer. The general time requirement per day is about 5 hours. Depending on the program you chose, your son might be able to do some of the offline work on his own while you were at work, and then you could work with him on other portions and supervise online access when you got home.
I think it's definitely a big commitment from the parents and the child, and it is probably not a good fit for everyone, but for some families and situations it might be ideal. I would search for programs available in your area, and once you find one or two that sound like a good fit contact them and see if you can get in touch with a couple of families using the program and go from there.
Good luck with the decision.