I'm not sure why, but they just don't seem to get it. Even after you leave them with the kid(s) for an hour, and when you return they're all "I don't know HOW you do it!!"; it still doesn't seem to sink in for very long. It sucks.
Try and keep that message in there a little longer by relating what you do, to what he does. For example, if he works in an office, say "Well, it's kinda like this...you're trying to type up a report for the boss; and your co-worker keeps coming in and erasing all your work by turning off your computer; how fast would you get your report done?"
That's exactly what it's like as a SAHM. You get the kitchen all spotless, and you feel all proud of yourself and accomplished inside. Then you turn around and you're face to face with your livingroom floor covered in toys. You then get to spend the better part of an hour "coaching" your older child to put the toys away where they belong, while you multi-task and nurse the baby. Then the middle child is hungry, so you put the baby in the highchair and give him/her some fruit and make lunch, ruining your beautifully cleaned kitchen. You turn around to find baby has smashed fruit into his/her hair and now needs a bath. Then you decide to take the kids out to the park to play. Two hours later, you're back home, and it's dinner time. While you try and juggle making dinner (turning the slightly messy kitchen into a disaster area) and minding baby; the older kids are busy upstairs creating what can best be described as a toy tornado......
I could go on like this all day.
Simply speaking, while you're busy finishing one task, two more are being created for you. It's like trying to walk UP the down escalator...those stairs just keep on coming; yet the top is SO CLOSE! Eventually, you just get so tired of trying to climb to the top that you stop, and you're slowly brought down to the bottom to start all over again.