Oooh, I love going to Germany, but it's not the most vegetarian-friendly country!
Most restaurants and cafe's will have a 'fleischlos' option. This means "meat free" - but NOT vegetarian or vegan (you will usually get some kind of seafood and cream cheese!) Look out for 'vegetarisch' (vegetarian) on menus, or try asking for "etwas vegetarisch." You might want to clarify this with your boyfriend's family, because they may assume you eat fish.
However, there are lots of sandwich cafe's offering health food, some of which is vegetarian or vegan. I've even seen Tofuwurst and Vegan Imbiss
The great news is that, thanks to German purity laws, you can drink all the lager you like