My first questions woulb be,
Is he an adult?
Does he have a good UV bulb?
Hot spot and cool spot?
What is his setup?

Adult beardies go through brumation periods. I have seen it last several weeks to several months. During this period they become very inactive, dont eat and will sleep in their hides(or buried) for long periods. This usually happens in the winter but I have seen it happen year round.

If the dragon has been healthy and is not loosing weight that could very well be what it is. I have had breeder females sleep for up to 3 months before breeding season.

I would keep an eye on here and see if its brumation or a lighting or heating issue.