Summon the dragon using the shout he gives you three times in tamriel (not necessarily in combat). He teaches you a new word for "soul tear" each time you do so

Just keep in mind that the summon shout works like a conjuration spell, you have to point where you want him to appear, and he can't only be summoned from so far away. I would recommend just pointing the summon shout at your feet and summoning him there.

As a side note, the shout doesn't summon bone men, it tears the soul out of whoever it hits, giving it to you, and filling a soul gem. The victem then becomes a minion for a short whole before disintegrating.