M.O.
It's normal to have a bm every other day - both of my twins (now 5 months old) are on that "schedule". As long as he isn't straining or seems in pain, he should be ok. If you're concerned, though, ask the pediatrician!
Here's my biggest reason for responding though. PLEASE, PLEASE PLEASE do NOT dilute the bottles - that adjusts the formula and is very dangerous for the baby. There was a Tribune newspaper article recently published about a baby that died because the mom diluted the formula to save money.