I have a two year old who will be three in the spring.. I just purchased her, she's completely untrained (well she learned to lunge this week)..
Anyway my trainer breaks horses at the age of two (about 16-18 mos to drive).. Morgans tend to be small too, but none of her horses have had any problems. In fact her three year olds are more sane, bombproof, etc. then most peoples horses will ever be. I would say weigh him (with weight tape) and use the rule of thumb that horses should not carry more than 20% of their body weight. If you weigh 135 (+20lb saddle or so) he should be at least 800lbs IMO.. Which at 14h, he could be.. I would say weigh him, and if hes within 15lbs of 800 or more.. then it shouldn't do any harm. I would just keep the lessons short and productive and not overwork him until he has the chance to fully mature.