My mistake, I see from Miminas post that you won't be residing outside of the country so technically you won't need to prove that you will come back..so I think you are fine just sending the papers from Serbia too..
If you had their national documentation and were actually a resident of Serbia then you would have to prove you will come back I believe.. I will let someone else answer that too...
When you get to your boyfriends place, he has 24 hrs to register your stay to his address (that is, at the police station or something similar) That will be your 3 month visa, the little card you get (all you need is your canadian passport).. Now since you plan to stay 6 months, there is no harm in going somewhere on a trip like to Austria or any neighbouring country with your boyfriend as a tourist in this case there is nothing illegal about that, and when you cross the border, the Serbian officials will take that little card that says you are registered.. Once you come back into Serbia, your boyfriend will just have to register you again, and when he goes he just says you guys went on a trip to Austria for ex. and they give you the little card again, and that is now a new 3 months that you have..
I was here since June, in July we went to Greece and when I got back we registered me again, so it renewed the 3 months... We just got back 2 days ago from Austria and he just registered me again today again so I have another 3 months from todays date.. they didn't ask me anything.. even if they did, its my husband, we take trips.. no harm
You can also apply for a prolonged visa like 6 months or something too, but I dont know much about that.. either way this was easier for us since we traveled a bit too...nothing illegal about it..