Hi.
I am so sorry that this dynamic is present in your famiy right now. I do not have the exact experience but I think I may be able to give you some suggestions.
First of all, In my experience, looking at a situation from another perspective is very helpful. I try to take myself out of the picture and look at it differently. Of course, doing this with family is very hard because of the emotion of famiyl and motherhood specifically.
Typically, there is a solution which may not make any one person feel "right" but can often give a little "win" to everyone. The focus, of course, is your son and keeping the focus on his best interest is priority. Having said that, the old addage "A'int nobdy happy 'til Momma is happy" rings very true. I am a single working mother and I make decisions that my ex and his family are very unhappy about. I must do what is best for me first because the directly impacts the girls.
Would your husband be willing to sit down with you and brainstorm the "ideal" characteristics of childcare for your son? This is not an exercise in getting rid of Grandma. It is a way for the two of you to each make a list then come together to see where the commonalities lie (you know, like couples do when they're deciding on a name for the baby).
Once you guys come up with a combined list of qualities, it may be easier for him to see you as a team. Try to keep in mind that Grandmas are trainable, especially with your husband's support. Don't think of her specifically when doing this exercise. Try to put your feelings about her and this situation out of it.
With that list, you can then look at the pros and cons of daycare vs Grandma-care.
**Keep in mind that research is done when looking for a daycare facility. Parents visit the facility and judge based on criteron and their guts where they want their child. The same should be done when a family member takes care of a child.**
So, let's just say that Grandma-care is the best for your family. Now there are limits, boundaries, and, yes, rules that need to be set in place. So, draft a contract. When children are placed in daycare there are handbooks given and signed. This is a contract. Parents agree to do certain things and so does the daycare. If one of the parties does not do what is outlined, then the contract needs to be revisited. Maybe things have changed and it's okay to let it go, or maybe it's time for a different environment for the child.
I know this is a long e-mail but I really felt called to respond to your request. Remember that this is NOT about your ability to parent. It's obvious that you're a good mom and you're being mature and responsible about the problem. In my experience, every time I kept my mouth shut, took correct and loving action, things worked out well. But, when I let my emotions run the show, it was like a scene from "Cops." Further, when I think of my advesaires as loving people just trying to do the best for my children then it softens my heart a bit too because I stop taking it so personally.
Finally, it may be that you and your husband need an objective party to help with this situation. There are probably lots of emotions at play. A marriage therapist can help couples get through such issues and it does NOT mean that your marriage is in trouble. It is a way to get help with a serioius issue so it doesn't damage your relationship. Rather, it could make the bond of marriage even stronger.
Good luck!