I'm in a very similar position OP. DS is 2y8m now and I just turned 40. I also have some underlying health issues, though not to the extent that it would be 'inadvisable' for me to get pregnant again.
However, DH is certain he doesn't want us to have another baby - at least biologically. He would potentially consider adoption. This is mainly because DS was poorly when he was born and was rushed to NICU and it was very traumatic.
I am really torn. As a PP said, I don't really have a physical urge to have another baby for myself, which really surprises me. I was desperate to have a baby before DS came along, but I think he has satisfied my biological urge to procreate. My reasons for wanting another one are that I don't want DS to be lonely, I feel that a family of 3 is a bit small, and as dramatic as this sounds, I fear that if anything were to happen to DS I would fall apart without another child to have to pick myself up for.
On the other hand, the thought of going back to the beginning is daunting. Financially I can already see the light at the end of the tunnel. We are all getting a decent amount of sleep and DH and I both manage to get some time to ourselves. Also my health did take a bit of a weird turn after having DS and i would worry about that happening again. Finally, at my age, I would worry about the increased risk of pregnancy loss or other complications. A big part of me thinks I should count my blessings and quit whilst I'm ahead.
Then again I think how much DS would love a baby sibling and how cute newborns are....
It's really tough! In my case I think it DH were keen, I would certainly do it. However he's not, so the decision in my case is whether I feel strongly enough to try to persuade him. And I'm not sure that would be fair on him (and I don't think it would work either...).
Sorry, this ended up being long. But it helps to write it all down!