Back to the smell of Boar brushes...I have two Semogue boar brushes -- one large and one small. Neither ever smelled at all. Long ago when I first wet shaved. I had a great brush that came with a mug and soap set from Aramis (throwback memory).
I wanted a second brush and got a boar from the drug store. When wet, it smelled like a badger shat in the house. I moved it to my workshop and use it still to clean loose debris from rifles. :gwh:
I think the best solution to try for your brush is the baking soda soak. It works on most odors in most porous substances. Follow with a hair shampoo/conditioner combo.