if you can handle taking him to small claims, you should do it. i've dated a boy (not a man) like this before and his promises are most likely empty. he just wants you to stop asking him. i think contacting the rental company is definitely a good idea, however, because the rental is in your name, am i wrong to think that this can be viewed as a theft? if he won't give the tank bank and technically it's yours (via the rental co.) then you may have the right to file a police report and let the rental co. know when (if) you have done that. you may not then be responsible for the costs.
good luck!