I can't help with youtube as I've never used it. But physical talks/lectures were always good for a few sales.
It might be possible for your husband to guest lecture at a few of the local universities/colleges and offer the book at the end of the session. Various deals are available with these people, he might even negotiate a fee for the lecture: he might also have to hand over some of his book sales. Perhaps talk to the humanities department (or whatever they call themselves nowadays)?
I also found that learned societies were always struggling for guest speakers. They rarely paid more than expenses (and a meal if I was lucky) but again, a few copies of my books would sell.
I think it's just a case of being creative who you contact to see what their interest is? (I was lucky, my field was business so there were plenty of openings and plenty of demand). Possibly your husband might find the more left-wing associations/societies amenable?
Something 'off the wall' like this might also be good
http://birmingham.truthjuice.co.uk/index.php/2017/01/terry-boardman-in-birmingham-on-280217/