It is a well-known fact that trolls regenerate. Their entire bodies are like livers. Cut one part, and it'll grow back. Pretty handy. Having the munchies? Eat your pinky and forgitaboutit.
Was this the first blessing the unicorn/mother had in mind when she asked the father to send the son to those toiling beasts? Who is to know? I suppose a better writer than I would have woven the answer somewhere. He would have built an intrigue out of this fact. The horse mother would have been depicted as a cunning (but pure) animal, with a long-term plan. It would have been good. But the opportunity's been wasted. If you wanted that to happen, it's too late. I'm not going to rewrite the entire story from the beginning just to weave that into the plot.
Besides, now that I've mentioned that it's something a better writer than myself would have done, how could I do it without looking presumptuous?
Where is this story going, exactly? Pff. This is ridiculous. The author doesn't even know where he's going with this. Let's just skip ahead to the part where the kid is already living with the trolls, building houses for those green beasties. Yeah, because trolls live under bridges, it's pretty handy to have them work the real estate business.
Unicorn Son was working the quarry. It was hard work. Moving stones, cutting and chiseling them according to exacting specifications. Sometimes, he'd cut himself on the edge of the stone he was working on. Other times, he would drop his mallet on his finger and say things so foul the pureness within him recoiled and tingled. It was hard work. Oh, wait, I said that already. Strike that from the record, please. He worked with the younger trolls, and blood and sweat mingled as they did their job. Using his horn, the hero of our tale was able to do a better job than most of his peers, working with such precision that he got jealous glances from his ugly friends. That must be why, one night, one of them snapped his mis-centered horn from his forehead, with little physical pain to its owner, but great psychological distress nonetheless.
However, the tears of the night were quickly dried when Unicorn Son awoke from his cry-induced sleep, and found that his horn had grown back! The jealous troll, on the other hand, was fuming. Fume and fume he did, until he spontaneously caught fire and roasted. He was served for diner, and was generally regarded as palatable. It must have come as a relief.
Fear not for the poor creature's fate: as I would have it, despite him not having been a particularly large specimen, there were leftovers that night, and from them he grew back in his entirety, not bitter at all from his experience. Oh, delirious yet merciful writer!
Now, the elder trolls were working on a very ambitious project. Kinda like the Golden Gate of the time, minus the bird droppings, I guess. Not that I've ever been to Washington.
All attempts to fix some large stones together to arch over the wide chasm had met with defeat. Countless carved stones had been lost to gravity, and sometimes trolls too. Trolls could regenerate but couldn't breathe underwater. I bet some of them wished they could have changed one for the other once they found themselves thrashing in the white waters that ran at the bottom of the chasm. Anyway, finding a stone big enough to span the entire width of the chasm was unfeasible, not to mention it would have been too heavy to move.
That's when Unicorn Son came up with a brilliant idea: drilling holes and fitting stones together with.. horns! And so they did. It took a few hours for a horn to grow back, so Unicorn Son was soon the subject of many horn snappings, for the good of all. Long stones structures were assembled, and the bridge building proceeded apace. It took three months to complete, and when it was done, Unicorn Son was the first to thread it, as he started the journey back to his homeland, his upper body strong from all the hard work (again with that! God, I really should find synonyms). He would be the first to tell the king his bridge was done, and the monarch would rejoice: finally, the men from the provinces would be able to visit the brothels of the capital!