He says, "Perhaps this is not the 'best of all possible' universes, but the only possible one. Possible worlds can mean only 'worlds that God could have made, but didn't'. The idea of that which God 'could have' done involves a too anthropomorphic conception of God's freedom. Whatever human freedom means, Divine freedom cannot mean indeterminacy between alternatives and choice of one of them. Perfect goodness can never debate about the end to be attained, and perfect wisdom cannot debate about the means most suited to achieve it. The freedom of God consists in the fact that no cause other than Himself produces His acts and no external obstacle impedes them -- that His own goodness is the root from which they all grow and His own omnipotence the air in which they all flower. (p. 26-27)"
I don't think that Lewis had Molinism in mind, I do, however, think that the problem that he brings up about "possible world" talk does point to a major problem with Molinism (as Molinism is founded upon possible worlds).