Many JWs, like religious people in general, can be selfish and rather arrogant and expect you to be the one to make all the changes and compromises. If you are not a JW then they may also be conflicted somewhat (not a good place to be) because they are not really keeping to the rules with what they are doing so may not really be serious that the relationship will ever go anywhere.
That.
If you go ahead with the relationship you will be the one that has to capitulate and compromise your beliefs to accommodate his. Bear in mind it's also a religion where women are subservient to men.
The fact that he has a relationship with a non-JW may indicate he's not strong in his faith and may be easily persuaded to leave but think very carefully before you go ahead.