Your holiday cottage is your second home, your haven
When you let family members or close friends stay there, it's because you are close to them, know them all well including their families and it won't feel like they're invading your space as you care for/ love and trust them.
When he invites friends he acting as if it's his second home and he decides who he can invite not you. All the downsides of your second home being commercially used and the risk, but none of the benefits of financial rent that it would gain. I doubt your insurance would cover it if they had an accident as you're accepting some money even. If a small amount.
I'd Start saying no or make the cottage unavailable when he asks.
We can understand why it's bothering you but I doubt he will as his mind doesn't see it the same way rest of us do.
He's misunderstanding as doesn't realise that it's your second home from home and is treating it like a social place for him and his mates to hang out for a free holiday, not your home. He wouldn't be allowed to invite extra ransoms to your home if you invited him over so why is he doing same to your second home from home?
Start saying no, if he asks, you're not comfortable with his inviting others to your second home. You prefer to stick to family only using it. (You don't need to tell him if your close friend & their family does too.., )