OK, going against the grain here but yes, I would contact AirBnb and expect some kind of refund. If the hot water system is unreliable then he should say so, it’s a basic amenity that can really affect a holiday. You have made the host aware of the issue and is "resolution ’ of using his shower wasn’t acceptable.
The host should have rectified the situation or offered a discount, it’s definitely not unreasonable to expect this. Contact AirBnb and point out you have tried to address this with the host to no avail. (Also worth mentioning the planned generator). A contract is valid if both parties stick to it, he has offered something substandard and not as described.