T.R.
I'm so sorry you are going through this...
Could it be an influence somewhere? Calling someone "stupid" or saying "shut up" is a *learned* behavior. And I know one thing...kids are sponges and pick up everything good and bad.
Are you calling him on the *good* things he does?
Maybe he feels a little shuffled in the mix right now and doesn't have the skills needed to get your attention.
It seems almost written and the fact that you know what is wrong confirms it.
Let him know your boundaries... "Honey, those words hurt and we only speak words that build each other up in this house"
Also make sure you have Blake time, take him somewhere special...just for him.... I really think he just needs his momma...... Maybe you can't change your childcare situation right now but you can change the dynamic of how he can handle the kids at your house. Have him feel a little more in charge......ei.. help with making lunches, u-know being more involved...like you two are a team. I think it will work wonders for you. God bless and I hope what I've said can help.