If i were you, i will get him another brother or sister. Coz you are concerning when you guys past away, he will be lonely by himself, having another family member can bring him more joy and caring within the family.
There's not going to be a problem on taking care of the 2nd baby while you are suffering now, coz what you need is to find a better and right ways of taking care a child.
I am a parent educational consultant, i see a lot of cases of parent finding difficulties with their kids, but when they know the skill and techniques, they then see alot funa nd joy having more kids in the family.
I have a kid of 2 years old, and i also want to get him another sister or brother, but unluckily i got divorced with my wife a few month ago, so i am not able to do so, and hope you find a better life and living way.
Express your own feeling to your partner is very important between you two. No good thinking in the way of your own thoughts always. Trust Me. !!:-P