If it cost $5 a month then he'd probably get three times as many subscribers. At least get twice as many, make the same amount, but double the audience. There's a psychological barrier at $5. Many people will spend $5 and think nothing of it, but $6-$7 starts to look like $10 and then you wonder if it's worth it.
What about the way Joe Rogan does it by mentioning sponsors only at the beginning?
"The world is a dynamic mess of jiggling things, if you look at it right." - Richard Feynman