Hi M.,
First of all, it is wonderful that your fiance loves your son like that. My dh is the same regarding my 3 children from another marriage, and it is truly a blessing for them to have a "Daddy" at last (their biodad is, shall we say, less than stellar).
Second, I'm guessing from the wording that perhaps it is financial issues regarding paying for the wedding. My dh and I had similar issues, and postponed our wedding from November of 2006 to February of 2007. It gave our children a good example of how compromise work in marriage. I really had my heart set on November, as I found the physical distance between us so difficult (he lived on LI before we were married), and he wanted Spring (after Easter). Just reassure him that you and df need those two weeks to get things more ready.
If, however, there are other issues that you and df haven't discussed / sorted through, please be careful about making those assurances in case you are unable to come to a compromise. Marriage is supposed to be forever, and an extra two weeks may not be what's needed. Remember, life won't wait for you to work through issues to see if marriage should continue - there will be difficulties after the wedding just as there are prior to it. It is not so important that you "solve" all problems for once and for all before the wedding. Life is full of problems that you will need to resolve and keep your marriage going at the same time. I guess what I'm saying is that if these things you need to sort out are the kinds of things you would ever consider divorce over - it might be better to not marry at all this point. My older 3 and I lived through a divorce (their biodad was an adulterer and wouldn't give that up), and nothing is worse, imo, for young children than that abandonment by a parent.
We will say some prayers for you.