I know excatly what you are going thru. I had the same problem 6 months ago.
My little munchkins are 25 months apart (girls).
She slept in the bassinet until she was about 3 or 4 months old. We just put her in her crib one night and it was that easy. I was also nursing.
If your oldest one is a good sleeper, I highly doubt it if he will wake up when the little one does.
There have been a few times when the baby did wake up and my oldest slept right thru it. Or, if the oldest one woke up, we just finished potty training, or had a bad dream, the baby slept right thru it.
Now, I can not keep them apart for the life of me. They do everything together. My oldest will not go to bed unless her little sister is in there with her. When she wakes up in the morning, the first thing she does is go over to the crib to see her.
If your oldest is fine around the baby during the day when he cries, he will be fine at night. Besides, you got the monitor, you will be able to hear everything.
Don't worry about a thing. They will be fine.
I hope this helps