"il est descendu" would be correct if the sentence was finished. But by adding a complement (he went down what? the stairs), we need to use avoir "il a descendu l'escalier". I know he didn't take the stairs in his hands, like he would with a case of wine, but it doesn't matter, the grammar is the same. "Monter" and "descendre" are not exactly like their English counterparts so don't over think it. Copy good sentences until they feel natural.