Agreed with above. ^ Now you could.... *fudge* the registration papers. That is not unheard of.
It is not unheard of to breed this late. My little buddy was born on September 4th of last year. He did not have baby buddies until he was weaned(earlier than we usually wean. We weaned him at three months only because you have never seen a more weather-hating miserable baby. Both mom and baby took it well.). And he had to deal with the miserable early winter as a little baby. I didn't like that much(neither did he). The stallion was moving out of the country, so we wanted to breed her to him for her first, so we ended up getting to her late.
Then you have size. When they are adults, it won't really matter, because they will all be the "same" size. If I were to want to show my colt before then, he would be way too small to be competitive.
Also with size, when it comes to training, you will have to remember that your two year old is not really two, so you will not want to do to much to the little one.
It is not unheard of, but if you are uncomfortable with it, you should tell them.