Probably not what you want to hear, but your best bet is to just practice sketching like that.
If you want the drawings to look more loose, maybe try to draw a little faster, and holding the pencil at a tighter angle (~30-35°). That's what I'd do, if I were to try drawing like that. Drawing at that angle will make the lines look thicker, and more messy.
Also, his drawings look like they're really small. They probably only take up 1/4 of the page, so drawing things smaller could help. It would be really difficult to make a drawing look like that with it taking up the whole page.
Another thing I notice in his drawings is that he seems to not use a super wide range of values. It looks like he sections off different areas to have certain values (kind of like those coloring books that assign colors to letters). So before you start to draw, on a different piece of paper, you could experiment with that. I'd say probably 2 darks, 1 medium, and 2 lights at most.