Honestly, there is no telling what he'll be good at until he gets more weight and you get him in training. Then you will get a feel for what he's capable of and you can go from there. Once the basics are down solidly, start introducing him to all sorts of disciplines or activities and one may really stick out. You likely won't want to run speed events or do any jumping for a few more years, as it can damage still growing bones and joints. I wouldn't base anything on his current condition or age - he's still growing and he'll be going through gangly, awkward phases interspersed with nicer, more balanced ones for at least another year or two.
If you wish to show before he's riding, I'd focus on showmanship. Showmanship is judged on how well you and your horse are presented, and how well you show an in-hand pattern. You can absolutely do halter classes, but he is a long way from being fit for a halter show. Halter is judged soley on the horse's confirmation per individual breed standard - essentially, looking for the best representation of the Saddlebred breed in looks, build, and body condition, including appropriate muscling. In-hand trail classes are great, but the majority of shows only offer it for yearlings; most assume you'll be showing in riding classes as a 2 or 3 year old. Regardless, you can still teach your horse to handle those in-hand trail maneuvers.
Good luck! He's coming right along.