Go to the Race-Tech.com website. You have to go thru their series of questions; just enter jibberish if you do not want to give out pers. info., but do be exact when it comes to weight, type of riding, and skill. Go beyond the Gold Valve promo, scroll around and you will find the stock and recommended rate (if you want to buy another brand) and their part number.
Just winging it the numbers mentioned above seem about right: .43s or so up front and a 5.6 out back should work for him but do verify with the website. I do not recommend he ride it with such light springs as it might headshake like a wet dog.
For about 1/2 price of new, Steve @ Motowest has a collection of used rear springs, or just about any other suspension shop. Just don't get pink or fucia (no joke, he has those colors). I believe the '89/'90 YZ/KX/RM uses the same fork springs (and KYB fork parts) as the '89/'90 RMX; check the online White Bros. catalog to be sure before shopping for used.