For washing out on the heel side, two things are coming to mind. One thing I often see newer riders doing is rushing the turn by pushing the back foot down the hill even with the front foot rather than simply edging. This results in a sideslip rather than a nice round turn shape. If this is you, then just be patient and let the turn come around naturally. Don't rush it.
The other thing is that you may be too far forward. Uncommon, I know, but if you're riding really far forward on your board, your tail will wash out. Try this experiment: Link a few turns while riding just on the nose of your board, then link some turns riding on the tail of your board. Riding on the nose, you may feel like your turns initiate really well, but wash out at the bottom (both edges). While on the tail, you may find that initiating turns is very difficult, even scary, but once you get toward the bottom of your turn, the back seat isn't a terrible place to be. So if this is the problem and you're trying to correct it by moving farther forward, you've got the wrong idea. Try moving aft as you finish out your turns.
As for your son, there are many things that could be happening. Where is he looking during his toe side turns? Are his shoulders countered to face down the hill? This can create some resistance to finishing toe turns, resulting in unwanted speed and a tendency to rush turns with ruddering movements.