hello...
according to the normal speech and language development pattern, children actually learn the 's', 'sh' and 'z' sounds around the age of 6 years. not necessary at 6 though, there are variations, but on an average around 6-7 years.
so far i dont think its something to get worried about, but yes, u do neeed to encourage your child to pronounce the sounds.
1. use the sounds clearly wen u talk as well.

2. reinforce your child even when he comes close to making the sound.

3. u should show ur child how u position ur mouth and tongue, etc. wen u make that particular sound. then ask him to do wat ur doing. eg. for 'sh' - lips are outwards, teeth are close together and the tongue is in between the top and bottom of the mouth, parallel to the ends. i hope u understand this.

4. use activities like speech making games, making shhhh sound, etc. to get the child to make the sound.

i hope this helps. :)

My son, now an adult, also had problems with the 's' and 'sh' sounds, and he also sounded 'slushy' (We call it "mush-mouthed" in my region.) And he sounded lispy as well. EXACTLY the same, at the same age.

I got him SOME speech therapy- like kindergarten and first grade. Then we moved, and he rarely had speech therapy after that.

Today, you'd think he sounds somewhat okay, but if you talk to him for about five minutes you'll start to notice that he sounds 'slushy', asking him to repeat himself.

I'd suggest that he get speech therapy ASAP and continue it for a long time.

I can hardly understand my own son over the phone these days. I have to ask him to repeat a lot.

Besides that, he's very smart- engineering mind! And he's always had a reserved disposition.

Anyway, I wouldn't wait on this at age six.

Hello,
I am a practicing speech therapist in the schools. First of all, it is great that you have noticed something atypical and are concerned about it. A lot of parents don't realize there is a problem until the child is older and it is much harder to correct something then.
The thing about speech sounds is there are different reasons children produce sounds incorrectly. With the /s/ sound itself, a child could be producing a phonological error in which they do not have a mental representation for the sound and replace if with others such as /t/. Because your child seems to be doing well educationally this is likely not the case. Additionally, though, there are still many other types of errors. If your child has a lisp it could be palatal, interdental, dental, or lateral. Palatal and lateral are very difficult to correct and are never considered a typical developmental error. This means it is important to correct ASAP.
Please go to a speech therapist to have an evaluation tailored to your child administered. Contact the school system to see if they will do it or look into your local hospital/private practices.
Good luck and thanks for being a concerned parent!
Michelle