Hi E.-
While I don't have any experience with weaning yet, I do have a nursing 22 month old, so I can definitely relate. I plan to let her self wean, but she also night nurses still and will freak out if I don't nurse her back to sleep.
Since your son is only nursing once at night, is it possible to let him come into bed with you at 5:30am when he wants to nurse so that you can all go back to sleep more quickly? I co-sleep with my DD and that is what makes the night nursing so bearable. I think I get decent sleep even though she tends to wake 1-3 times a night (sometimes more if teething). I too am hoping to get pregnant soon, but will continue to nurse like you are.
I really think if that is the only feeding bothering you to try the co-sleeping thing. If you are against that I suppose your husband could take on the roll of soothing him back to sleep.