If having your grandson remember the trip is a concern to you, I'd probably wait until he was seven or eight years old at a minimum. My twins are now eight, going into 4th grade, and they barely remember things that happened in kindergarten. My own personal memories start in earnest in 3rd or 4th grade. But heck, 6 years is a looong time to wait for a trip to Disney World. What about you remembering the trip? That's important too.
My oldest daughter (now nearly 12) doesn't remember many of our early Disney experiences with her. She doesn't remember how she was nervous about the characters and then ran out of my arms to hug Dopey because she loved him so much. She doesn't remember when Minnie Mouse pulled her out of the crowd to have a special ride on Dumbo with her. She doesn't remember her first trip on Space Mountain. Or the time she buried her dad in the sand at the Grand Floridian. But those memories are priceless to me. And they also set the stage for all the great Disney fun that she does remember.
Two years old is most definitely not too young to have a fabulous trip to Walt Disney World. You can see an answer I gave another guest about this
HERE. I say the best solution is to take your grandson now, so you'll have the memories of this special experience, and then take him again in a few years when the memories will be his own.