Your case is not simple, but it can be solved because of your intention to get married. One of circumstances for getting temporary residence permit for foreigner is marriage with polish citizen. 
 
At this moment the best solution for your fiancé is to stay in Poland and start procedure of legalization his residence. On the one hand it is necessary to get all required documents for getting married, on the other hand to apply for getting temporary residence permit to Governor according to foreigner residence.
 
It should be also mentioned that Law on foreigners as a rule protects persons living both in informal relationships, as well as in the formal relationships from deportation in a situation when it would undermine the right of foreigner to live  in the family.
 
For example, in art. 330 of Law on Foreigners it is said that foreigner can get residence permit for humanitarian reasons or permission for tolerated residence, if there are reasons to provide it or residence permit if foreigner is married to polish citizen, or to foreigner who has permanent residence permit or long-term residence permit in Poland  and don&rsquot object to the reasons of national defense, or national security, provide of safety, and public order if only the purpose of marriage or its existence is to circumvent this law.
 
Furthermore, according to art. 330 of Law on Foreigners a decision obliging the foreigner to return is not issued to a foreigner who is married to polish citizen, or to foreigner who has permanent residence permit or long-term residence permit in Poland, and don&rsquot object to the defense reasons, or national security, provide of safety and public order if only the purpose of marriage or its existence is to circumvent this law.
 
The cost for leading this case is 5000 zł &ndash half (2500  zł) is paid upon signing the contract, the half (2500 zł) after the legalization of your fiancé&rsquos  residence in Poland.