D.M.
I know that this isn't the kind of advice that you're wanting to hear but in my opinion it would be best for your son if you waited until he turned 18 and was out of the house before you started seeing someone, and especially before you start making plans to get married. By bringing another man into his life, you are making him feel much less important, especially at the age of 10. The statistics for 2nd marriages (esp. w/children involved) aren't good. You haven't known this man very long...not even a year. Why are you so willing to sacrifice your son over this relationship? How would this affect him down the road (say 3 or 4 years) when you and this new guy decide that you're incompatible and divorce? How does your son benefit from having a new dad, and in particular this one? Are you thinking mostly of your son when making these decisions? Or are you mostly caught up in the good feelings that come w/the newness of dating?
Again, I know that this isn't a popular opinion, and probably the last advice that you'd want to hear but I really believe that these are all things that need to be considered before deciding to date or marry. I wish you well on the decisions that you make and hope that you really do make your son an important part of the decision process, and not just an afterthought that you need to train to accept your decisions.