Maybe it's just the lighting/angle.
Not sure if serious because of the angle jokes people make about formerly huge BBs in "now" pictures but no, the volume of hair there is clearly low. The faux-hawk and shaved sides look with a short beard is what most men who have very little on top do before they turn to shaving.
Just look at his hair back in the day.

And now

It's just aging and DHT affects him too. RIP.
There were briefly photos of him on bb.com where his hair had been dyed completely brown (or not bleached blond, whichever) and was brushed forward. Wish I could find those photos. They showed how thick it looked.