OP, I'm glad you are planning a heart to heart with your DS. I caution you to be very careful in how you word things with him. Try not to bash the FDIL or her family. Speak to the circumstances as it effects your DS as much possible. If you talk badly of your FDIL or her family, you run the risk of alienating your DS.
1.) I'd tell him real love can wait. I'd tell my DS to be realistic. They are living with her parents, planning a grand elaborate event. They would be better off to get an education, find a good job, save and be prepared for the marriage than rushing to pay for a wedding with money they don't have. I'd tell him to think about waiting, and postpone the wedding. Encourage him to find a place of his own, and work on a secure future. It would be better for him and FDIL (although once he's away from the influence of that family, he may see more clearly). I'd rather have him ready for the future, any future, than living with money grabbing, overbearing people such as this. Ofcourse I'd leave the last part out, he'll find it out for himself and you won't be the bad guy.
2.) I'd tell him I love him. Ask him if he had a friend in this situation what he would advise him to do. Surely, he would see that he should make life decisions without future inlaw pressure.
3.) I agree with telling your DS to use protection from an "early" pregnancy. I'd do it from the standpoint that they don't have their own place yet, it would prolong him (them) having to live with the inlaws.
ETA- 4.) If he does not want to postpone the wedding, I'd tell him the wedding they are trying to hold is not feasible. I'd remind him I'm not rich and didn't win the lottery. The wedding plans and the money they want to spend on that one day would serve them better going towards a future home and security. They could have a smaller scale wedding (without the ostrich feathers and such) that would still be very nice. (Even if the princess and king and queen disagree)
Good luck with the talk.

I hope it goes well and at the very least he will agree that the direction they are headed needs his involvement & input (and less of the FIL).