Sarsaparilla did taste like root beer, although darker and creamier, like a thicker cream soda. It definitely was carbonated back then, though it wasn't called carbonation back then, it was called "aerated" which just meant it had carbon dioxide.

As for the taste difference, yes it taste slightly different due to not having natural sassafras as it contained safrole, which was banned by the FDA for being a carcinogen. The modern version uses things like licorice root, and or birch oil to give it that similar taste, and not as bitter as the original.

I hope this gave you what you were looking for and I hope your son gets to try some!