"I am 30. Had a son at 20. Always wanted more kids but I've been busy with school and was just married to (not the father) last year. Now I'm (finally) ready for more kids, but my son is not. He says "You will love them as much as me!" and cries. How does one answer something like this? Any ideas would be great, thanks!"
If I were my MIL or Dr. Laura or a hard-liner like that, I'd say something like, "It doesn't matter what your son thinks. You're the parent, and if you want to have another baby, you should do it."
But I'm not a hard-liner, so instead I'm going to say that your son's feelings matter very much, and if you want another baby you should have one.
Siblings don't always get along, and yes, it's possible that your son and this baby won't end up being best friends or having much in common as adults. But that doesn't mean the experience of being a big brother won't be fun or worthwhile for him, or that it will ruin his life. And what if this sibling becomes the most important person in your son's life? There's no way to know until they're both a lot closer to grown up.
It sounds like your son is used to the way things have always been. The marriage itself must be a huge thing to adjust to, and he's probably already wondering about his place in the new family (no matter how great his relationship is with your new husband). So thinking about being "displaced" with a new baby can't make him happy.
But his feeling of belonging within the family is a separate issue from whether or not you should have another baby. I'm sure he knows this intellectually, but it wouldn't hurt to reassure him again repeatedly that you want another baby precisely because he's so great and you know a new baby will be, too, and not because there's anything wrong with him, or that he's no "fun" now that he's practically a grown-up. A new member of the family is going to divert some of the attention from him, but it's also going to create more love and more fun in the family.
My advice once you do get pregnant is to involve your son in as much of the pregnancy process as possible, but without attaching any expectation for his participation or enthusiasm. Talk about pregnancy, birth, babies, siblings, etc., but don't expect him to be interested or happy about it. Let him talk about his negative feelings and acknowledge them without judging them. As long as he knows that he doesn't have to like or love the baby as long as he doesn't hurt the baby, he has the freedom to form his own relationship with the baby without going through you or your husband as gatekeepers of emotion.
I'm going to plug, once again, the amazing book Siblings Without Rivalry. The central idea of the book is that there's no way you can force your kids to like each other, but you can treat them as individuals and make sure they know how to resolve conflicts with each other. I read it while I was pregnant with my second and found it to be helpful even then in framing how I talked to my son about the sibling he was about to have in a few months.
FWIW, the new baby will probably idolize your son. It's a ton of fun to have someone who thinks you're just the best thing ever. I'm sure your son isn't thinking about it now, but when you have another child your son is going to get an adoring fan who wants to be just like him. He's probably been thinking in terms of sharing you instead of in terms of getting yet another person who thinks he's the greatest.
Good luck with building your family. Your son may not be thrilled about it initially, but if you can make sure he knows how much he's loved and you avoid comparing your children, he'll enjoy being a big brother. Most of the time, at least.