It all sounds wonderful!
And, yes, attachment, as in wanting and expecting things to stay the same, wouldn't be healthy nor practical. But I see no reason to hold back affection and caring (if that's how you see becoming "attached" to him).
You actually don't know that it will hurt, if and when J. finds someone else. If you tell yourself it will hurt, you could make it a self-fulfilling prophecy, but... where's the fun in that? It might hurt, but it might be something beautiful... there are other possibilities. You just might experience compersion and be happy for him. We always project the worst possible scenario when looking ahead or experiencing a rush of feelings. Why not project a great scenario?
If, after some time being involved, J. got a fantastic job offer that would take him out of the country and far away, wouldn't you be happy for him even though you would miss him? You would want the best for him, so it would be no different in a love relationship. If he wants a primary relationship and finds someone to love in that way, wouldn't you be happy for him? You also don't know where that could lead... A new friendship for you? A quad, maybe? This is all speculation about something that hasn't happened yet, anyway. If you hold yourself back NOW from feeling the joy and pleasure of what you have -- for fear of being hurt later -- you won't truly be present.
I have some friends who always say, "Live today like it's your last." That doesn't mean we don't plan for the future, but it does mean that we never know what will happen, so enjoy every precious moment now. Our time with the people we love will always be limited.