Sounds a little like one of the phases one of my kids went through. Our child started calling my husband by his first name. It was only a few times. It wasn't out of disrespect, it was just that THAT is what they heard ME call him all the time. That was his NAME. I quickly started using Daddy when I spoke directly to him, for a few weeks anyway. We didn't make a big deal over it. And it didn't last long.
If he is sandwiched between two other sibs, maybe he is somehow (in his mind anyway) differentiating between what HE wants and what the other kids are doing? But more likely he is learning diction and proper grammar usage.... Do you ever say to him "Mommy can't do that right now." or "Let Mommy finish __ and they I can do that." Anything like that? Well, to him, your NAME is "Mommy"... he may be emulating that, trying to figure out when it is appropriate to use his name when referencing himself. Kinda like the really really little ones say "Me do it." before they learn to say "I".
Each child will have his own quirky sayings/doings. Write it in his memory book and you'll look back on it fondly in years to come. :)