drago has two scenes. sicne I don't know exactly which one you mean-
His first scene is in the first tavern you stay at, the one where the coliseum fights take place I think? All you have to do is have a few point of affection with him, then check on him at night. he'll invite you inside, then later ask if you wanna have fun.
Doing so unlocks a foreplay scene between you and drago, but you get interrupted by thane.
The second scene happens later, at ruadens castle/temple/whatever teh hell it is in the swamp. if you've unlocked dragos first scene and shown interest in continuing what was interupted (I forget where the choice is, but if you just hang out with drago as much as possible you should find it easy) He'll meet you in the hallway at night and invite you back to his room to finish what you two started.