It turns out being undead may have some perks after all, at least when it comes to giving pleasure to his boyfriend (is Sirius his boyfriend? They never really did sit down and have that conversation, did they), but the feeling of a mouth around his cock when it's him giving the pleasure is something that takes a moment to adjust to, but once he does, all he wants to do is make Sirius come.

Although...

He eases back, enough to speak. "I can't swallow," he reminds Sirius, knowing he'll have to spit, so now he takes his time at the head of Sirius' cock, lavishing attention there.