Are you taking enough time to take care of yourself? I know that when I neglect my own needs I have less empathy and patience for my kids when they get hurt or need something. You already know, but you can hear it again: Make sure you let yourself be number one very often, especially now, during your pregnancy! 
Â
Your relationship with your son will change some, but like a PP said, when your baby is born, you will discover a new depth of love for your first child. I had a definite adjustment period, a mourning period, I would call it, for the singular relationship that I lost with DD1 after DD2's arrival. Every mom has some guilt, I think, but it works itself out, especially if you go in with an open mind and an acceptance of all of the feelings you have for the situation. If you have misgivings, honor them and integrate them. I wish someone would have told me that!
Â
The first day DP went back to work, the cat woke both kids up right after he left. I was still on a serious hormonal roller coaster after the birth, and I just sat on the bed with 2 crying babies, bawling and asking myself, "What have I done??" lol That was one time. I tended to both kids and gradually learned how to balance them, and now, of course I can't imagine how I would cope if I just had one kid and she didn't have a sister to entertain her! Their relationship is absolutely the most priceless, beautiful gift anyone could have bestowed on them. No one understands you like your own siblings. Their friendship alone is worth every second of hardship and new adjustment that we go through.
Â
Don't worry, it will be a beautiful journey and, just like after your first birth, you'll find a well of love that is unfathomably deep.