That is very unhealthy. There are feeds/supplements designed to give a young horse all it needs in a very small dose, so they can live mainly off forage, which is what they need. Too much concentrate is bad for ANY horse, let alone a baby.
This is from page 3 of the NUTRITIONAL ASSESSMENT OF WEANLINGS
AND YEARLINGS paper done by Kentucky Equine Research.
I find that this argument has gone beyond the scope of the OP, but felt that this information is still relevant. However, this will be my last post on the matter of concentrate/hay ratio.
I just thought I'd share what all the equine research facilities have been proving. It's up to you whether you want to believe them or not.