JW life is challenging, so it may be hard for outsiders to understand why someone would inconvenience themselves so. Why do they?
The fact is, you can't unring a bell. Once a person accepts that the Bible doesn't teach Trinitarianism (one of the easiest Bible truths to grasp), he may be convinced that 99% of Christians are brainwashed by that big lie. For many Witnesses, the hundreds of other doctrinal differences with Christendom just reinforce the conviction that tradition, convenience, and personal preference mustn't be allowed to outweigh Bible truth, at least for a principled person of faith. Anti-JWs tend to attack personalities and incidents, but anti-JW theological arguments just don't ring true to these folks.
Just as a person may avoid good nutrition and exercise during periods of his life, a JW may have periods of relative inactivity in the faith and may even have unhealthy binges of what the faith (and he, perhaps not even fully acknowledging it yet) believe to be "sin".
It sounds like the boyfriend/fiance is such a person, currently but temporarily inactive. It seems likely he will eventually become more active in the faith, and adhere more closely to its beliefs and practices. He will not celebrate holidays, and he will seek to teach his children his faith. He certainly will NOT force anyone to get baptized! He certainly WILL be under powerful obligation to be a respectful husband and conscientious father.
My advice to you is to marry him, he's a catch. One caveat is the likelihood that in time you will realize it is the truth.
- (1 Corinthians 7:10,14,16) [A] wife should not depart from her husband... and a husband should not leave his wife. ...For the unbelieving husband is sanctified in relation to his wife, and the unbelieving wife is sanctified in relation to the brother... For, wife, how do you know but that you will save your husband? Or, husband, how do you know but that you will save your wife?
My advice to him is to remain single until he can find a mate with whom he can be "evenly yoked".
- (2 Corinthians 6:14) Do not become unevenly yoked with unbelievers.
- (Deuteronomy 7:3,4) And you must form no marriage alliance with them. Your daughter you must not give to his son, and his daughter you must not take for your son. For he will turn your son from following me, and they will certainly serve other gods