There is a 6 year statute of limitations on contracts in Wisconsin, so you can sue him for all the back rent that he owes you. As for other costs (food, utilities, etc.) if you didn't have an agreement with him to pay that, then you won't be able to sue for that.
As for the money paid for the improvements, if there was no agreement for you to repay him, then you can claim that it was a gift from him to you to help pay the costs. He will claim that it was a loan or that it was conditioned on him living there. But without any formal agreement stating you would be responsible for repaying him, he will have an uphill battle getting a judge to force you to do so.
The only other argument that he may make is based on "unjust enrichment". This is a legal theory that means when a person unfairly gets a benefit by chance, mistake or another's misfortune for which the one enriched has not paid or worked and morally and ethically should not keep. I don't think this would be successful either as he voluntarily made those expenditures and it was not chance, mistake or misfortune that caused him to do so.
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