I admit my experience is very limited, but I do think it is hard to tell until the horse is actually hooked to the cart. And even then, it can vary depending on what class you WANT them to be in.
Here is a photo example: This is Princess (Aloha Acres Fashion By Magic) trotting free in the pasture with no one chasing her:
So, you think maybe Single pleasure, right? WRONG!!! Here she is actually driving in the ring, granted it is late in the day (dark outside) and we are both tired, but still not atypical:
This was a Pinto show where there is only one choice for driving, but in AMHA, I have entered her in Country Pleasure and would seriously think about the new Classic Pleasure class for her. (Could a pro trainer get her to fit into Single Pleasure? Maybe, but I am quite happy with her the way she is.)
So I would wait to see how the horse looks when hooked. I would also be very anxious to get your boy hooked, because I do like the way he moves.