Okay, I'll give it a shot, he has a false ewe neck which is also too skinny and short for his body, REALLY long, sloped, weak pasterns, a long back, a not-so-nice topline, and overall, just NOT stallion quality.

Another thing is the purpose of breeding your mare. You should NEVER breed your mare because you want a kyoot ickle foalie woalie, which is esentially the reason you gave. It makes you sound uneducated, especially when there are literally thousands of cute little foals already here who need a home, most of which will die of neglict and malnutrision. Now, you seem like an inteligent person, so I don't think you really thought about it from that perticular angle, an easy mistake.