Hi F.! You might want to try to go back to just nursing the baby. That sounds like so much work to me to pump and feed the baby a bottle. I think it is easier to just nurse the baby. Try that for awhile and see if it helps.
The other thing I would suggest is laying down to nurse. It is more relaxing for you and your milk will let down easier. That way the baby will get more milk. With two little ones you need all the rest that you can get.
The other thing that worked really well for me was to read my older child a book while I nursed the baby. That way everyone was relaxing while the baby was being nursed. My older child called it "mommy time." Both he and the baby were getting "mommy time."
With a second child it is easy to get "busy" and really not feed the second child as much as you fed the first child. Second children are typically less demanding than first children when it comes to nursing. Nursing is basic supply and demand. Your body will produce whatever it needs to produce to feed the baby. If the baby is nursing often it means that your milk supply is low and nursing often will increase your milk supply.
My older son was always off the chart...way over 100th percentile. My second child was off the chart on height and 50th percentile on weight. My oldest son has always been tall and built solid. My second son is a tall, lanky 17 year old now. All kids are different.
If you are concerned about the baby's weight gain make sure you are doing these three key things--eat a healthy diet, drink plenty of fluids (no caffeine because it dehydrates you), and get plenty of rest. Your hubby can wash the dishes, put in a load of clothes, and pick up your 3 year old's toys. It is important that you go to bed early and get plenty of sleep. You need rest for your body to produce enough breast milk to feed the baby. Take care of you and make nursing the baby your priority for the next year.
Enjoy your little ones!