As far as age differences go, well I think anything goes, and ANY gap will have it's pros and cons. Too close, two in diapers, two who want to beheld at the same time, but they will most likely be very close too.... Too far apart, jeluose sibling, your used to being a little more independent nowand you go back wards, but older child can help, they will definitly be thier own people, they got to be the only baby for awhile... Pros and cons all over!!!
As far as your son is concered and the whole marriage or no marraige thing goes, well I have never met my "biological" father either and I was raised by my Father a man who has been in my life since I was less then a year and who we lived with my whole life, but didn't actually marry till I was 13. Honestly I didn't care, if he already understands that you guys are a family that is all that matters to him. And all that will ever matter to him.
Good luck,
H.