I know it's hard but have you made time to do something just you and him. My daughter was 21 months when we had our second daughter, and I think it does go a long way to make sure that I spend time with the oldest, just me and her. I always read, just the two of us, before bed. My husband works nights, so even if that means once Brooklyn is fed, in jammies, and has a clean diaper, if she has to cry for about 5-10 minutes I've been doing that (she's a bit older now though). But when she was younger, as much as I may have wanted some sleep while Brookln napped, I used that time for me and my toddler to have alone time, reading books, watching a movie, running around the backyard....whatever she wanted to do within reason.
Also, try to involve him when you can. Are you bottle feeding? Encourage him to hold/help hold the bottle for you. Have him help out with diaper changes by getting a new diaper for his brother. Or bringing a burp cloth. A great way to "keep him off your back" while you are feeding the newborn is to sit and read with him. It'll be good bonding time for all three of you.
I hope this helps a little. I'm sure you'll plenty of great ideas on here. Most of these women are awesome!