A.F.
I'm not a single mom, but I'm going to give you some advice that has served me well in a lot of difficult situations.
"Start where you are. Use what you have. Do what you can."
-Arthur Ashe
Honey, I felt like I was cheating my son for some reason when I found out I was pregnant with my daughter. I bawled like a baby! :) Part of that is irrational pregnancy hormones, so don't worry! Your son will adjust SO fast to having a sibling- you aren't taking the spotlight away from him, I promise!
Get a baby sling. You can wear your baby while doing activities with your son. All of you will benefit! :)
You are giving all you can to the son that you have and you will be giving all you can to this next baby as well. You sound like a mama who wants to give the BEST- and you will.
I hope you have a good support system! Being a mom of two is hard work, but you can do it!