To improve speech it is important to serve as a good example for your child. For example your child says “tone” but you know he means “stone”. In that case repeat the word, stressing the avoided “s” sound, so say sssssstone. Make corrections always in a positive way, by repeating the word once “yes, that is a ssssstone”. Take care that a child does not get frustrated by too many corrections, be subtle in your way of correcting. If you use the word “stone” in a context you can also pronounce it in such a way that it is clear for the child that the parent pronounces the word differently from the way the child did. In case of continuously replacing or avoiding sounds or inaudibility, speech therapy treatment is recommended.