While you may think his baby speech is cute, don't say so to your child or mimic his pronunciation. He may stick to it even after he doesn't need to. Don't correct his words, either. You don't want to make him any more self-conscious than he may already be. Simply model correct speech.
Lisping occurs when children push out their tongues when forming an s instead of letting their tongue rest behind their teeth. Most children outgrow lisping eventually. But if your child's speech makes him unintelligible or prompts teasing from other children, you may want to see a speech pathologist. In general, the younger a child starts speech therapy, the better the outcome.