I remember when I was pregnant with my second child, my midwife and I were chatting during our wonderful hour long appointments, and she asked me if I was afraid of transitioning to two children and that taking away from the first, since another client of hers was struggling with this. I told her, and stick to this day, to the idea that: The greatest gift I can give my child, and the thing that only I can give her, is a sibling.
I grew up with one sister, and we had some rough spots but honestly I don't know how I would have made it through without her. I know others are perfectly happy with their only child, and that many only children are perfectly happy, and that some sibling children are unhappy, but for me it was clear from the beginning that children in a lot of ways need each other. I actually had hoped to have more than just the two girls...and my dh (at the time agreed)...
But a few weeks later he had an affair and within months our "family" was a "broken home" because he was too selfish. I read this thread when it first came out, and didn't post...but once I read your description of your husband, I got so many chills, because really, if someone is selfish enough to compete with their children, they are incapable of putting the children first. I remember that my ex had a slinky in his office/our basement, and he wouldn't allow any of dd's toys down there, but she saw this one and was interested in the many times I took her down to do laundry (like you, I did most of the work, and had to make ends meet financially as well). He got so mad that I let her play with "HIS" toy...it was not something he was willing to share. And I was just floored by that attitude...because there was nothing of mine that was worth more than my child's happiness, not one thing.
That selfishness manifested in his affair and determination (yes, determination, he refused counseling, refused to end it, etc.) to end our marriage and I think it was just when I was becoming more independent/making things work around his stubborn selfishness that it fully manifested. (after years of working part time to make up enough for us to not be on the streets, I started doing in home childcare and training for doula-ing and childbirth teaching, and was finally making enough to keep us afloat+)
As you saw this same sort of scenario happen with your parents, I'd ask you this...what if you DON'T have another child, and stay with him, and he one day just leaves you and your son? How would you deal with the regrets then? How would you feel about all the sacrifices you'd made? And heck, he'd probably be able to get alimony from you as well, since you make so much more, plus half time with your child, and suddenly you, who'd never done anything to NOT put your child first, are spending holidays alone. Because though none of us want to imagine these as possibilities, it is right outside the door....
I know I'm bringing a lot into this, but since it happened to your parents I'm sure it's not foreign to you. And since you've noticed that your dh's selfishness has taken form since your child was born, then it's even more likely that the me-first attitude will take hold before not just another child, but both you and your child.
If I were in your shoes, I'd try counseling, to truly get all these issues on the table. Perhaps it would help give you perspective on what's making your dh tick, how he feels threatened by your commitment to mothering, etc. And perhaps he'd learn a little more about how unacceptable it is to compete with a needful and helpless child. And maybe having some perspective would help you work through the choice to expand your family together, rather than as a power struggle.
But there is no way that one spouse should sacrifice everything in a marriage when the other spouse is determined to take and demand and drain life away. That isn't a marriage, really and truly it isn't, it's a hierarchy where the giving spouse is on the bottom. I found it helpful to read Shel Silverstein's "The Giving Tree" and see what he projects as the future for those who give all in hopes of an occasional affection...it really offered me a lot of perspective.
sorry you're faced with such a difficult situation, but you'll probably make it through this as a stronger person and mother no matter what you choose, so long as you exercise the reflection that you're showing here.