I would not advise starting up your own business with a small baby. I run my own firm and it's purely and solely because I have 50/50 shared custody that I'm able to do it. Well, I could do it with a full-time live-in nanny/au pair I guess. It also takes a HUGE amount of mental energy which you're not going to have with a new bub.
I think your boss's offer is fabulous. I would work 3 days and have him pick up the childcare bill. I worked 3 days a week when my DS was six months and it was great for me.
Plus it's best to go into a job that you know and can easily do, when you have a new baby. It's not the time to be taking great leaps of mental agility or having to expend masses of energy learning new tasks.
Childcare is not the devil. Look around you at the adults you know - can you tell who was in care and who wasn't? Thousands and thousands of healthy, happy, well-adjusted children and adults have been in full or part time care from a young age.