This is how I'd compare the pianos if I were to buy one (from the scale of happy, to VERY happy): Kawai RX-2, Yamaha C2, Kawai RX-3, and Yamaha C3 (even though it's not on you list, but I think since you have Kawai RX-3 on it, you should consider Yamaha C3.)
personally, I prefer Yamaha action more than Kawai, but I like the mellower tone in Kawai more than the bright Yamaha tone. If I were to choose between action and tone, I'd choose action, because granted, Yamaha's on the brighter side, but it's still beautiful, but key action is a very personal preference to each pianist. So, if your son's the one who will be playing the piano, and since he likes the Yamaha action more than Kawai, I'd say go with the Yamaha. To me, I find Kawai Rx-2/3 action to be a little stiff. However, you do get longer, better bass piano if you choose Kawai, especially, Kawai RX-3. That's the reason why I throw in a Yamaha C3 for you to think about. If you could find one around your area within your expected budget, I'd definitely buy a C3.