The problem isn't the 's' or the 'l' sound; he's perfectly capable of pronouncing those sounds correctly in other words. When I try to reproduce the way he says it he tells me I'm saying her name wrong and shows me the correct pronunciation, and will only accept when I say her name correctly. I think it's pretty cute, but then I'm the mom.
That ended today. He looked at me while we were in the car driving to pick up Sylvia from school, and after pronouncing her name his normal way he said, "No, Syl-vi-a." Like a little language lightbulb went on in his brain, and now it'll stay lit forever.