If your boyfriend comes to the US legally from Germany on the visa waiver program without a visa and you get married can he stay here and change his status Will they make him go back for any reason?

This was not advisable course of action in my opinion. However, he should be able to adjust status by filing the I-485 and I-130 (and I-765 if needed). It's advisable not to leave the country until he receives his green card (even if advance parole is granted). http://www.dixonimmigration.com/index.php?pid=2