Tiff,
I best advice here is take your time with all of this. When I met my current husband I had a daughter who was seven and he had three children (11, 9, and 6). We dated for 2 years before we became engaged and then didn't marry until we had been together for three years. And now we have been married for almost two years and while I am happy that we are married, it seems that we are still coming across things that we didn't discuss prior to merging our families together. And even when things seem great when you are not together all the time with the kids, all of sudden when you are, emotions erupt over things that you didn't think were previously problems.
And then yes of course, money for a family of 3 or 4 children is significantly different that a family with one. Just think about the multiplying bills, every time you go somewhere and buy something for your son. Just a quick trip to McDonalds can go from $10 for two, to $30 or $40 for 6. School clothes, supplies, shoes, etc...
I'm in no way trying to rain on your parade or say that your relationship is doomed. Because if he is the one that you are suppose to be with, you will find happiness with one another. But if there is no impending reason for you to get married quickly, I urge you to take things slow. And really go over how you both plan on handling all kinds of situations with your children, where is everyone going to sleep, how you are going to handle holidays with families, who is going to discipline who ?, etc... The first couple years of marriage are difficult for people with no children, but adding in a couple can really make things interesting.
Good luck!