Without formal training the best thing you can do is stabilize until ems or someone trained gets there. As bad as it sounds if it's not a life or death situation immediately the less you do the better (there are exceptions of course). I think you did great just trying to help and going through his injuries mentally that quick. There is often more going on internally that you can't see so that why I say stabilize and wait if help is that close.
im im sure you had the gear for it but doing surgery on the side of the road probably wouldn't have been a good time for either party.