I can relate as well! I have a 5 month old daughter and a 2.5 year old daughter, who was a dream eater, sleeper, you name it. My baby was/is colicky and sleep has always been an issue with her. At 3.5 months she was usually going longer than 2 hours between feedings, but she would regress back into "newborn behavior" every once in a while and be up 2-3 times a night sometimes! Even now, though she's only 5 months, she's up once a night like clockwork. She is a pretty big girl as well, and eats pretty well, but her sleep routines have always been tough - hard to settle her in, hard to keep her asleep. It's tough if you first baby wasn't like this. My older daughter was through this phase by the time she was my younger daughter's age.
She did the grunting thing at night when we'd settle her down as well, and swaddling didn't help her either. The one thing we have found to help her is a sleep sack, actually! I think she loves the warmth it gives but she can move around in her regular pajamas and not be restricted by a swaddling blanket. We also try to hold her and spend time with her before bedtime so she is drowsy, but not totally asleep.
I can also totally relate to working full-time and operating on little sleep. I am in the same boat, and went back to work when my baby was 3 months as well. It seems like whenever you finally get into a deep sleep, you are woken up by the baby, then you get the baby and yourself settled, and it's time to get up again! That's how I spent my entire summer! I wish I had advice for that problem; I'd take it myself.
I have heard that Enfamil has some rice in it so it fills babies up better than other formulas. I'm thinking of trying that myself. Keep staying the course with your son; he will get there and you'll be so happy you don't have to start from scratch again. Babies are so immature at 3 months, but when you are up all night it seems like they are so much older! I understand! Good luck and hang in there...