It's the sex, of course. Even "worldly" kids don't think seriously about getting married that young. JW kids feel they have to marry to have it. A parent sound like a satyr to suggest anything else, I suppose.
I don't know all your personal circumstances, and what risks you assume in being straightforward with him. One thing I would mention though, is the danger that should he marry a JW girl, then his odds of staying in increase a lot. Yes, 50% of marriages end in divorce, but don't forget that 50% of them DON'T end in divorce. You are risking the lives of your grandchildren as well.
I was a born in who left before baptism, stayed out for 16 years, and then got all invigorated because I resumed bible study with a sister that I liked. This was followed by thirty years in "the truth" most of which I was vigorously active. I raised my kids in it, and my one son and daughter-in-law have spent their entire lives in full time service and have not had children due to their zeal for the work. She is probably too old to change her mind now, and it grieves me to know that my ignorance is the cause of this situation. Most people who knew me during the sixteen years I was out would NEVER have thought there would be a day when I would be so IN! You just never know what the future will bring.
It is for this reason that my sister is going to discuss this with her own daughter, who is a "casual" JW. Who knows whether her grandson will be casual or zealous, wll attend or not attend college etc, or will marry someone like that? She feels that she must take the risk.
It is because I don't want my son and daughter-in-law to spend any more time in a work and life that is not for and to the purpose of truly serving god, not an organization that I intend to speak frankly with him. I gave most of my life to this. It has to stop here. My son is an elder, holding two of the top positions in the congregation, serves as the head of the hospital liason committee, is a full time pioneer, and a former bethelite, is active on the body of elders of the RBC committee in our state, and is a convention overseer. He is the golden boy of the circuit and district, and the darling of the CO and DO. Given that, I expect that I will be quickly disfellowshipped. I am going to do it anyway. He is my firstborn, my child whom I dearly love, (as much as all my others who will certainly leave) and I have to do this as his mother. I don't feel that I have a choice. It is a moral obligation as far as I am concerned.
All that being said, EVERYONE IS IN DIFFERENT CIRCUMSTANCES!!! These are only my own thoughts and opinions and are no better or worse than anyone else's. I only offer these as something to consider in making your decisions.
You have to do what you think is right in YOUR circumstances, given the personalities and inclinations of everyone involved.
The very best of luck to you. I hope you let us know what you decide, because I learn from the experiences of others, as we all do.