Hi--
My son had similar issues with weight gain. It's hard to understand the need to wake a baby up at night until you've experienced the worry slow weight gain brings.
Once his weight was firmly back on track, we increased the time we would let him sleep (4, 4 1/2, 5...). We stopped setting the alarm around six weeks. However, we also decreased the time between his daytime feedings to make sure he got at least the same amount of food in. That will also help your milk adjust.
Good luck - When this started for us, someone said we would always have issues getting him to eat. He's almost five months, and if anything is out of sorts, he isn't interested in eating. For us, things definitely got better, but I'm still waiting for the worrying to end!