Control and stability at speed are the main advantages of longer skis. Fore/aft balance in unconsolidated snow too. I'm not going to speculate any more on specific proper length other than to repeat the observation that both my upper intermediate ex-wife and my advanced but not expert current girlfriend ski daily drivers in the mid-160's. Most of us think that your height/weight would at least offset their ability edge.
The appropriate ski may well be different for you and your son. Both of you need to be observed by a qualified instructor to get the proper advice.
The appropriate ski may well be different for you and your son. Both of you need to be observed by a qualified instructor to get the proper advice.