I do NOT like surprises. I would HAVE to tell my kids at least a couple of weeks before leaving. For us, part of going to WDW is the excitement leading up to it. We enjoy watching disney movies and doing a disney countdown calendar. We even plan what outfits we'll all be wearing while there (Disney ones, of course).
As far as RNR, just say "we'll measure you when we get there". Personally, we don't do "lift" shoes, but one pair of NORMAL tennis shoes raised my DS a whole 1/2 more than the others. Still don't know why, I think it had more padding - which is why we considered it in the first place - all the walking!!!
Let him know that you'll be doing the other parks first and enjoying many, many things on your trip. Get him excited over Expedition Everest!!! I believe it has a shorter requirement than RNR. Also, if he doesn't do "well" on it, then RNR should not even be considered.
Also, just for the record - it is VERY VERY easy to avoid RNR. Yes, mickey bars are sold at the crossroads for TOT and RNR. It's difficult to see RNR from the ice cream stand, but stand with your back towards RNR just in case.
When my DS couldn't go on it (hopefully, this Dec will be his first time he can), I just explained that I didn't think he'd be tall enough, but we could check after we got there. If he wasn't, he could choose anything else for he and I to do (we gave some options). (He couldn't get enough of TOT!!!)
Be sure he understands, it's completely a safety issue, not an issue of him "being old enough". Of course, he's "big enough", just not tall enough YET.
We also allowed our DS to make the decision if our older DS and dad could ride without him. We took the attitude - if he can't ride, we won't ride, no big deal. Our older DS felt badly for his little brother and was willing to wait. Of course, little DS wanted his brother to ride even if it was without him.
Whatever you do, if he isn't tall enough, make sure he does something special during that time. Do not allow him to just wait outside the ride!!!! It's a CONSTANT reminder that he's not tall enough. We also split up prior to the street going to RNR. It was easier for DH and DS to leave us then the other way around.