It's not a good idea to come to the border and ASK for 6 months stay. You'd have to show proof of funds to support yourself and they'd harass you like crazy. The way it works is this: if you're from a visa-exempt country and you come to a Canadian port of entry and are admitted, you're automatically authorized to stay for up to six months. What you have to do to be admitted in the first place is convince them that you only intend to stay temporarily. Having a boyfriend/fiance in Canada is a "red flag" to officials that you might not leave, as is not having a job at home, or a place to live, etc. So, first, when you attempt to enter Canada you should have a plausible explanation for why you're coming that does not include "living with" your bf for six months. They'll ask the "purpose of your visit": have a suitable answer like you're coming to holiday at a friend's cottage, or coming for someone's wedding, etc. AND they'll want to know how long you plan to stay - a few weeks maximum is reasonable . . . plans can always change once you're here, as long as you don't stay longer than six months. So, if you're going to fly, it's best to get a round trip fare with a return about 3 weeks later, and the ability to change your return date if you need to. Once you're in Canada, you can just call the airline and change your return date. I actually did this a couple of times - not because I had intended to but because my daughter had this tendency to get sick when we came to Canada to visit, with an ear infection or a stomach flu that made it impossible for her to travel - and we'd call and postpone our departure date for a week or two and that was fine. But I also successfully entered Canada by car before Christmas one year, telling the IO at the border that I was going to stay through the New Year, and because I was driving a vehicle licensed and registered in the States, he let me in and I easily stayed six months with my fiance.
However, looking back, I was lucky. I'd never try that now because I had no proof of my job or anything with me at the time. It would have been a long trip home. So - be prepared . . . a return trip ticket is usually sufficient to convince them. If they "suspect" you, though, you'll need more - like a letter from your employer or a rental agreement. (You and your folks could draw up a rental agreement, just get one at a stationary store and back-date it, and throw in some amount for rent - although, if you have a return trip fare and a job letter, you probably won't need more.)
Bottom line: remember - even though a visa-exempt national is authorized to remain in Canada for up to six months after being admitted . . . that's not the same thing as having the right to ask for a six month's stay on entry to Canada. You may know that you absolutely do not intend to stay longer than six months but you will NEVER be able to convince them of that - so don't get into it. Secondly - a relationship with a Canadian means you are automatically a risk of overstay . . . so don't get into that either. Have a plausible explanation for your visit to Canada - and reasonable plans for return. Once you're admitted, you can stay for up to six months - and then you need to leave, or apply to extend your stay. Be aware, though, that an application to extend will only be approved, basically, if you happen to get married while you're in Canada and you have an ap in process for permanent status. If that happens, come back here for information on how to successfully extend your stay. It will need to be done BEFORE your initial six months status expires.
Just on a personal note - a bit of unsolicited advice: staying with your bf in Canada for six months might not be the best answer to his "jitters" about getting married. It's really stressful on a relationship when one partner is out working and the other is stuck at home (in a foreign country especially) with nothing to do all day . . . and then there's the burden of additional cost for food, utilities, etc., that he'll take on having you there. Maybe a couple of months every once in awhile would be better - after all, they always say "Absence makes the heart grow fonder". Before you decide to marry this guy - give yourself the chance to make sure his heart grows just as fond of you in your absence as yours does for him.