My husband wanted to stop after one too. He's an only child, and I have 2 sisters and a brother. I didn't feel like I was "done yet" either. We had several discussions about it.
Our issues were slightly different in that we had to have help getting pregnant to begin with too. He mentioned to me at one point a long time ago that he noticed that my siblings and I had something he didn't ever have growing up too, but couldn't describe it - just something he felt like he missed out on being an only child.
He agreed to "go for 2" but wasn't real "gung ho" about it - just he knew it was that important to me. We do have 2 daughters now, and wouldn't trade them for anything. AND, the second is a true "daddy's girl" through and through. If I try to go get her if she wakes up in the night, she will bat my hands away and fuss until my husband comes to get her, or I promise to take her to daddy.
Our daughters are 3 years apart, and get along great. Both my husband and I are 42, and we have a 2 1/2 year old and a 5 1/2 year old. So I know what you mean about the age thing, but it can be done!! And with the "wanting to retire when he's 58" thing...has he seen the economy lately? Chances are he's not going to be able to retire at 58 even if he wants to just because there are too many variables that are beyond his control (like the economy, the stock market) and unless he's found some new way to grow money trees, I wouldn't be counting on that "retiring at 58" thing unless he wants to go work at Wal-Mart when he's 60. (Not trying to quash the dream of his, just trying to be realistic.)
The best thing I can suggest is to have a true heart to heart, and explain to him how important it is for you to try to give something to your son (a sibling). Help him to realize that when the time comes for him to make decisions about his parents (if they need to go into a retirement home or something) that at least he has siblings to talk to about it, and with you, it's just you. If you have 2 kids, at least you won't put the whole process on your only child. (These were all things we discussed too.)
Realize too there will always be a part of you that "wants more". I loved being pregnant with both my daughters - LOVED it. I can almost understand the Dugger family having 18 kids--I loved being pregnant that much! I know we decided on 2, and that was "it" - but I still kinda wish we could have had more sooner.
For what it's worth....and good luck!!