As one of Jehovah's Witnesses he really shouldn't even be dating anyone outside the congregation either since dating is a precursor to marriage.
tacosalad is correct, that is the reason we do not marry outside our faith. The scripture that really elaborates on this is 2 Corinthians 6:14
"Do not become unevenly yoked with unbelievers. For what fellowship do righteousness and lawlessness have? Or what sharing does light have with darkness?"
What this means is that our beliefs are not compatible with other religions. A good example from the bible is king Solomon. He married wives that worshiped pagan gods. In time he too started to worship them and was punished for it. By marrying only in the lord we don't have to worry about distractions that may lead us astray.
Our perception and understanding of God and Jesus is different than the Catholic standpoint. We don't believe in the trinity, hell, that most of us will not be going to heaven, so on.
Ask yourself this: What will we teach our children? What are we going to do about holidays? (We don't celebrate the holidays.)
If you were to study with the intent of becoming baptized then it would be OK for you to date. However, you would not be able to get married until both of you are baptized witness.
I am surprised that the elders haven't counselled him about this. Unless that is they don't know.....
You are correct in your last question. 'Do JW see other Christians as unbelievers?', we see other religions as false. 2 Timothy 3:1-5
"But know this, that in the last days critical times hard to deal with will be here. 2 For men will be lovers of themselves, lovers of money, boastful, haughty, blasphemers, disobedient to parents, unthankful, disloyal, 3 having no natural affection, not open to any agreement, slanderers, without self-control, fierce, without love of goodness, 4 betrayers, headstrong, puffed up with pride, lovers of pleasures rather than lovers of God, 5 having an appearance of godliness but proving false to its power; and from these turn away."
I suggest you both go to your respective spiritual leaders and ask for their advice. The longer you let this play out the more heartache if things go south. I wish you the best in dealing with this situation.