Welcome to the forum BNG,
You think your situation is unique but in actuality, it's all been done before. Regardless of the details on how you met, in the end, you are a regular person interested in in a relationship with a cult member. If you are truly at ease with the situation, you would not be asking advice and feedback from strangers on the internet.
To a JW, Jehovah (actually, the organization and its Governing Body) take precedence over anything or anyone. Even if you become a JW, you will always take a lower position regardless of his claims.
While he may tell you he would never attempt to convert you, if he remains a JW, he is expected and required to do so. JW men are considered 'Head' of the family and he will be expected to have his family in subjection to his authority.
Even if you do not join the religion, do not expect celebrations most people look forward to (Christmas, Birthdays, Valentines, Mother's Day ...) will be happy occasions. While you may wish to celebrate, decorate, entertain, visit family and give gifts and cards, he will likely have no part of it and may even disallow seasonal decorations in or around HIS home.
Upon marriage, he will most likely act as your power of attorney if you should be injured and not able to speak for yourself. As JWs believe blood transfusions are wrong, he will most likely oppose you receiving a blood transfusion even if your Drs state you will die as a result.
Many people raised as JWs will stop attending and may lead a life which is contrary to JW requirements. While they may assure their partner they will never return to it, unless the person recognizes the religion is not the 'Truth', too often, they will return even if away for decades, particularly when there has been a life-changing event such as the birth of a child, death of a family member or life-threatening illness. Although they may appear to have left the cult, does not mean they no longer believe the cult's indoctrination. While you may think you can prove to him the teachings are not 'the truth', he may resent you for even making an attempt, stating you are being influenced by Satan or worse, apostates. Each JW must come to the realization that JWs do not have 'the truth' for themselves, but many never do as they are usually unwilling to even consider it.
Although you think his family are not as strict because they continue to associate with his brother, it could be his brother was never actually baptised into the religion and was therefore never technically a JW.
Just because his parents are aware of your existence, you really do not know what they have been told about you. Perhaps they were lead to believe you are interested in becoming a JW (the outcome will outweigh the method).
While JWs claim they are the only true Christian religion, their beliefs and practices prove otherwise. This and many other forums discuss the real truth and background about the JW religion, warts and all.
While probably not what you wish to hear, for your own benefit, please accept we understand how JWs think and we also recognize what is too likely to happen if you continue pursuing this relationship.