Congrats! I am getting married in October. :-)
As far as having an outdoor wedding goes--you are right. Weather is very unpredictable, but my sister got married in June and they had an outdoor wedding. I however am not for a couple reasons. 1) Who knows what the weather will be like in the fall. 2) I have several older adults at my ceremony that probably would not like to be outside and deal with horrible weather.
My sister got married on the lawn of the building where her reception was--sounds cheesy but they were out in the country so it's not like they were on a busy street watching cars go by or anything. If I wasn't getting married in October, I might consider doing the same thing as well for my wedding. The one thing you want to be sure is, that if you decide you want to be outside, make sure you have a definant back up plan on where you can have the ceremony take place instead in case of bad weather.
Why does your fiance want the ceremony outside? If he thinks it will save you two money--think again. Sometimes all the rental of the chairs, tent, etc. will be just as much as an indoor event. But if he likes being outside, explain to him that weather is even more unpredictable in the fall--you have to worry about rain, cold weather, and worst case scenario--snow (yes, I wouldn't doubt it in some places).
Maybe you could do what my sister did and have your ceremony where you are having your reception, and do it outside. That way if you need to, you could have your ceremony inside. Plus your guests won't need to travel too far to the reception. :-) Best of wishes.