First off, if you mean 20 and 1/2 hands, it would be written 20.2 (a hand is 4 inches, and the 2 is 2 inches, or 1/2 of a hand.)
At 5, how tall is he currently? Some horses (drafts especially) DO grow a bit after 5, but he must already be gigantic if the estimated finished height is 20.2 hands. 20.2 hands is HUGE, even for a draft horse, and it is especially tall for a Percheron as they are not one of the taller draft breeds.
The current (as of Sept. 2009 I think) tallest living horse as recognized by Guiness World Records is 20 hands tall.
They said he was 18 hands at 18 months old! Remington claims title as world's tallest living horse | Horsetalk - International horse news
Here he is with a 5 foot 8" handler--
There is another horse named Poe that the owners feel is taller (unofficial measurement is 20.2 hands) but he has not yet been measured by Guiness yet--
Both of the above big guys are Clydesdales, according to the news articles.
Soooo, if the big guy you are going to see DOES top out at 20.2, he may be in the running!
When you go get pics (can't wait!) can you measure him, or estimate current height? I have a little spot by my basement stairs where I drew 4" increments so I can tell how many hands tall I am (or my chin is, or my collarbone is, or the top of my head is, etc. ) wearing various footwear-- for example with my everyday workboots on, a 16 hand horse would be as tall as my eyebrows. (Yeah I know, the wall chart is a bit obsessive, LOL)
As far as my tallest horses--
I raised a Percheron/TWH cross gelding that made 17.1 hands as an adult, and he was taller than either parent at maturity. (TWH dam was 16 hands, Percheron sire was 16.3.) I currently own an Appaloosa mare who is 16.2+ and I have owned two other Appaloosas who were 16.3.