12 year old SS

Hiya looking for some advice and or also a little rant. My step son hates to keep him self clean. He's going to be 13 in a week and not sure if it's just an age thing as I don't have my own children as of yet. He comes over every other weekend and I'm very close to saying "If you don't wanna shower then don't". Twice he has been caught running the shower water leaving it on for a bit and then lying and he had one and it's really f***ING annoying coz I'm the mug that's paying for the bill. It's not just himself, he struggles to keep his room organised and clean and it really grates on me. It's hard to put strict rules in place as he only comes over every other weekend from Fri - Sun. He lives an hour and half away so shared custody isn't really an option. His mother has the same issues at home. He started coming to us about 3 years ago on a continous basis before it was just ad hoc thing because his mother kept him away from us. We have had 3 failed transfers looking to do the 4th this year and I don't want the added stress trying to deal with him god willing this transfer works. Feel like I'm stuck between a rock and a hard place and could really do with screaming into a pillow.