I actually just had my 4.5 year old evaluated for this same lisp in the last few months. She was evaluated by the Easter Seals program through her preschool and also through our local intermediate unit. Both evaluations showed that she isn't expected to master the "s'" sound until at least age 6. She scored well in all other areas so was not considered to have any language deficiencies and therefore wasn't eligible for corrective measures.
The IU evaluator indicated that since she is capable of placing her teeth together and can make the "s" sound if she really concentrates (she played games to get my daughter to do this) and since she makes the "j" sound appropriately that she will likely outgrow this lisp. She suggested playing games with my duaghter like having her put her front teeth together and "hiding" her tongue while working on the "s" sound.
In discussing the issue with her preschool teacher she told me that her son had the same lisp and she was told that once he gets his adult teeth the lisp may resolve itself. It did.
I exchanged emails with the speech teacher at the elementary school my daughter will attend. She said she goes into the K classes once a week to work on language skills and if the lisp isn't corrected by age 6 or 6 1/2 then she would be eligible for an more thorough evaluation through them.
So my plan is to see what kind of progress I can make with her and take advantage of the Easter Seals evaluation again when she it comes to her PreK class next year.
I know it is frustrating. My advice would be to go for an early intervention evaluation to see if he qualifies for any services and to get some tips from them on how to work with him. If it still really bothersome you can always seek out speech therapy services that you pay for. One other piece of advice I was given was not to make it seem like a big deal to my daughter to not put stress on her or make her feel inadequate. Best wishes to you!