Question: Will my son be Jewish if I do not get him circumcised?
Answer: Now this is a question I do not hear too often. You are engaged to a Native American whose beliefs would forbid the circumcision of any son the two of you may have together. You ask if your son will be Jewish if he does not have a circumcision.
There are no easy answers to your question. I'm not even going to begin to answer this question from the perspective of traditional Judaism -- there are too many issues involved: the circumstances of your conversion, the father of your existing child, your hypothetical son's status, etc.
From the perspective of Reform Judaism, I would say that you and your fiance need to make a decision about your children's religious identity. Painful as it may be, the two of you need to decide if you will raise your children to be Jews or followers of your fiance's tradition. It is possible for them to value and cherish both, but it is not possible for them to BE both.
Children who are raised in two religions generally grow up feeling like outsiders in both. Religions are systems of belief -- either you are inside them or you are outside of them. Children crave an identity. Parents do them a disservice by denying them a sense of belonging when they need it the most.
If you decide that your child is Jewish, let him or her be a Jew. If he is a boy, give him a b'rit milah that will give him a visible connection to the Jewish people and the God of Israel for the rest of his life. If you decide to raise your child in his or her father's tradition, let your child fully embrace that tradition.
It's a tough choice, but, as you know, parenthood is filled with tough choices. This will be the first of many to follow.
Rabbi Jeffrey W. Goldwasser