I have been riding bareback for years, on all ages of horses. Although yes true, without the tree of the saddle your weight is not distributed half as well, but you also weight less.
Like Orchid said, if he doesn't seem uncomfortable, he will be fine. Horses seem more responsive because you are paying better attention to what you are doing. Also you can feel every movement, as well as they can feel yours.
Oh and I do everthing bareback, walk,trot, canter, gallop, jump, barrel race, rein (which is very hard!) etc. Its all in the legs! Keep in mind your balance gets a lot better when you ride bareback......as it takes balance and strength to stay on!
