It tends to be a bit of a wive’s tale that restricting feed changes the growth. Usually what happens is the foal and/or mare become deficient and then you end up with a foal who has joint problems. With warmbloods feed problems can result in the dreaded OCD problems.
Especially for warmbloods (is this what the foal is?) genetics dictate that they grow rather remarkably fast for the first 2 years of life. It can make your eyes boggle out of your head! Really and truly. This can make feeding them rather challenging. I would never restrict feed in a horse to change the rate of growth. Foal will land up where he/she is supposed to, based on what their genetic code dictates.
All you can do is ensure the foal gets exactly the minerals, vitamins, protein that they need to ensure the growth is correct and stable, so their bones, joints, ligaments, and tendons are sturdy and uniformly correct.
You can have a 2 year old who is already at 16 hands and then they seem to sit there or grow very slowly and eventually top out at 16.2 hands, for example.
Others grow like a steady-eddie and still land at 16.2 hands.
I have had each of these cases repeatedly over the many years.
There is difference between feeding them sufficient and force feeding for growth. You want the first and not the latter. 