waitwaitwaitwait... a hot spring you say? hmm... I did give him a flower and a tank top as a gift after the second heart but no mention about a hotspring? Gonna recheck it later...
So far I know the event isn't repeatable, unfortunately... Would love to be proven wrong..
EDIT: Ok I checked already, he mention that if you want to put a fancy decoration somewhere then you should've visit the hotspring instead of the sawmill... and then your MC is teasing him, so I don't think it was a legit invitation from him :/