I am dating a illigal alien, he is here illegaly. If we get married can he still be deported, and can he become a legal resident without going back to his country?

If he entered US illegally, he has 2 choices: voluntary leave or deportation. In both cases, if you're married and want to file a spouse visa or not married and want to file fiance's visa, he needs to get a "waiver" and get approval first. He must wait for visa number to become current in his home country. No other option.

If you just get married he won't get deported, only if you file for him to be legal and then more then likely he will have to go back till all the paper work gets filed