Severus scowled as he read Harry's owl. He briefly wondered how long the fool boy had left it this time. He put the letter down and turned his attention back to the class he was teaching. The Fourth Years were quiet, concentrating on the potions with only the odd one or two flicking their gaze towards him.
He picked up his quill and scrawled a quick reply, sending it off then tucking Harry's letter into his robes. He got up and began to stalk through the classroom. Harry did have something a legitimate question about Faustus but it would be easy enough to lock the door to his bedroom and ward it to tell him if Faustus needed him in the middle of the night.
As for explaining why Harry was there, that was easy enough. He'd already explained the nature of his relationship with Draco... and hadn't that been a hideously awkward conversation. Faustus had seemed to accept that even if he didn't entirely understand the whole thing. Severus knew that understanding would come as Faustus got older. His son didn't need to know the true extent of what happened between he and Harry. Simply explaining that Harry needed a safe and secure place to sleep and that the wards around his rooms were one of the few things Harry trusted would be sufficient. Faustus was intelligent enough to know that there were some things he didn't need to know about and that he shouldn't talk about his father's private life.
Harry,
Of course I am still willing to help, idiot boy. This can be explained in a manner that Faustus will understand without revealing anything.