I'll probably get some negative feedback for my suggestion, but I think that he would be better off with an RX6. True, the RX6 is able to handle far greater abilities, but I think it would be a good ski choice and one that he could enjoy more and advance more with. There is a substantial difference between the RX4 and the RX6.
If you were to go into a good, honest ski shop and ask them for a beginner ski, they would point you to a section of the wall and say "anything there." Most beginners are skiing at a price point for skis, boots, bindings, and poles. At the RX4 price point (a good ski at that point) skis are designed differently than at the next higher price point. Any ski needs to have some torsional rigidity so that it doesn't flex/chatter all over the place each time you turn. The lower price skis do this by making the ski stiff - everywhere. It is stiff (somewhat) torsionally, but also stiff longitudinally. This makes it more difficult to turn or if it is easier to turn, it chatters when it does. When you get to skis in the RX6 price point, you will find a ski that is stiff torsionally, but soft enough longitudinally to allow the skier to make easy turns.
I started skiing after the whole shape thing started. I bought a ski comparable the RX2 or RX4 and had a miserable time for the first couple of years. At a sale, I bought a ski a little more advanced - similar to today's RX6. The first day out, I found my standard blue incredibly easy and I spent the rest of the day on steeper blues and blacks with little effort.
I really think new skiers often start out with a budget so limited that they can not enjoy the sport.