You don't have to feed him between 9PM and 5 or 6AM? Wow! Wish my kids slept like that at 4 months!
I think you are doing everything just right. Does he start to surface from sleep for that 9PM feeding? If not now, he will sometime soon as that gets to be his routine.
The medical community is unambiguous about solid foods. Every medical organization recommends that a child be EXCLUSIVELY breastfedfor the first six months,so no water, no cereal, no nothing except Mama's yummy milk. Following this advice can greatly benefit your son's health both in the short term and for the duration of his life. Another 8 weeks of letting him be a baby isn't too hard is it?
Can you figure out a way of giving him his tub in the morning when he gets up early? Then you wouldn't have to wake him for that. I know you probably want to spend more'awake time' with him when you get home, but that will happen soon enough as he needs less sleep.
Maybe nursing him at the breast for that last feeding would work for you, too. As you probably know, a pump will never be as efficient as your baby at extracting milk from your breasts and keeping up your supply. Maybe he wouldn't be as upset at being awakened if he has Mum's body to soothe him.
Best luck...sounds like you're doing the right things and asking the right questions.