m not experienced with hip dysplasia, but I gather
German Shepherds often get this. I hope someone else posts who has more experience, and can help you more.
It is typical of bone pain, and/or tendon pain to be worse at night. Don't ask me why, but it is. But of course if he HAS just pulled a muscle, it could be more uncomfortable after resting on his bed in the evening, for example, then getting up to climb the stairs.