INCREDIBLY thin hair

First post. Used to keep my hair long and brush it forward right from the back and sides which I know was stupid but knew this moment would come when I shaved it down and thought bloody hell .... what I'm left with is the most thin hair halfway through my scalp. It's almost like the dht has started at the front and gone towards the back like a typewriter if that makes sense. Anyway is there anything that can make these thick again or they are just waiting to fall ... pic attached