Well, there’s a sense in which this very line of thinking is a basis for believing in the Trinity.
Instead of just saying, “no, God can’t do that,” you might try to help him think about what “making another God” would look like:
For this “other God” to be truly God, the “other God” would also need to be eternal. So the “making” would have to have gone on from all eternity.
And since by definition there can be only one God, the “other God” couldn’t be “another God,” but would have to share in the same nature as the “first God.”
This is what orthodox Christianity believes about Jesus. We don’t call it “making another God” because “making” implies something that had a beginning in time and “another” implies a different nature. But of course a six-year-old isn’t going to be using language so technically, and I think in a way he’s on to something that may help him understand the Trinity.
My own elder daughter is nearly six and thinks about this stuff too. . . .
Edwin