Is it crucial that you go back to work? Your son is already expressing his concerns, and I think you need to trust his judgment. He's not ready to lose mom to anyone, especially another male (the alpha male, at that!).
Is your income really crucial for your survival? Your son doesn't need a room of his own, tons of acquired stuff to hold him, entertain him, and to keep him occupied, he needs his mother!
A baby's need for his mother as his only caregiver lasts for at least two years, probably three, and best for five! That's what allows a child to grow up trusting and loving.
When we leave our babies, it's perceived by them as abandonment! You have only a few years with this baby, your son, and this is your time to help him to become the best he can be--to thrive--not a time for him to be developing skills simply to survive. We do, of course, want our children to thrive...and that means being there for them.
Your son is telling you, in the best way he knows how, that this is HIS time. He NEEDS his mother. It's your job to be there.
And, by the way, this part of the mothering job is short-lived, it's a window of opportunity, and, if it's missed, you've not done your job and your son's life will reflect the loss of his mother too soon.
Sorry, I know this is tough, but it's crucial information and, now, you must determine your priorities in your life and in the life of your child. You can't compromise on this one!