You can measure before really getting in line. Even if there's a line going beyond the measuring stick, I've found that just taking one adult and the child in question, and walking past the line to the stick, and if you feel like it using hand signals to show that you're just measuring the child, works.
It's going to be very important that the child knows how to stand "big and tall", but without going on tiptoes. Practice practice practice! And every single time you get into a line, tlak about how you are going to "see if" they can ride "this time". If there are really important rides, try at the very beginning of the day. That's when we're all the tallest, after sleeping and rejuvenating all night.
NOw with Grizzly, that might be chilly, but it's a GREAT ride! So bring lots of changes of clothes (down to underpants and socks and shoes...don't forget the shoes like I did when DS got soaked elsewhere, lest you end up buying a $30 pair of Buzz tennis shoes b/c no one wants to walk all the way back to the hotel), put them in a backpack, put the backpack in a free locker near Grizzly, and go on a few times in a row. Make it worth your while, if he's tall enough.
If he is NOT tall enough, and there are others wanting to ride, a word of warning. The two attractions inside of the Redwood play area are also 42" attractions.

Even more important than bringing dry shoes is to NOT go in there, expecting that a kid can go on those things (wall climbing and a ropeswing sort of thing) to take his mind off of Grizzly, not knowing the height req is the same. There are OTHER fun things in there, don't get me wrong...but if there's a height stick, run away.
We too had a "paper above the head" moment. On Star Tours, at the second measuring stick just before you head to the seating areas. We hadn't practiced standing big and tall, and I wasn't there and DH blanked. The CM was also a big UNcool, because he put his hand on DS's shoulder, which caused him to hunch a bit, and DH didn't think quick enough. CM put the paper between DS's head and the stick, and it was all over. (meanwhile a tiny girl wearing tall cowboy boots, completely not tall enough, walked by and the CM didn't notice, which is why I'm against *purposely* buying tall shoes)
So the child needs to know to stand big and tall. Otherwise the line will have been for nothing (though actually the lines are mainly pretty cool, especially for a kid that age).
FYI, in case you don't know, Grizzly has only lapbelts. And for a little kid, you have to cinch them up like crazy. And you can get wet from ANY angle, even from below (the water pools, then the raft hits in a funny way and the water can shoot up from the floor), which can be shocking for little kids. Don't get me wrong; it's one of my son's absolute favorite rides (on one trip we rode 10 times in 2 days), but it can be surprising, especially with the lapbelt, when they are likely still used to 5 point harnesses in moving vehicles!