M.B.
J.,
What if you CAN'T get him sleeping through the night? What will you do?
Try a swing - if you can't find one that will move him (if he's over 36 pounds) mayb ea baby hammock. He's developmentally delayed, he's going to need the extra attention as you go along anyway.
Don't worry about doing it WRONG, worry about what he needs.
Mostly I'm not a believer of letting them CIO. If your husband isn't either, then likely it isn't for you. Easier to have him there in the room for you/DH to respond to faster so he will get back to sleep faster.
Sleep begets sleep. Any time DS has missed a nap, it has totally messed with how much he got sleep the next day. When his nap is LATE bc we are out and about - that doesn't help either.
The less YOU stress over getting him to bed at night to stay, the easier it will be on YOU. Nap during the day with him.
Do you rock him at all?
Congratulations on your new baby-to-be, and his older brothers. You are one great Mom!
Good luck,
M.