My son Jack had speech therapy through EI for 8 months (November '06 to July '07) that he "graduated" out of before aging out when he turned 3 last July. We pursued speech therapy because by age 2, he had minimal words and NO 2-word phrases, and his inability to communicate effectively (aside from grunting and pointing) was frustrating for all of us. But he did very well with ST and we were very happy with the progress he had made when ST ended. Ever since, he's continued to make great strides with his vocabulary, usage and enunciation/pronunciation. But my husband and I have noticed over the last couple of months that he struggles with beginning "S blends" by omitting the "S". For example:

"snow" comes out as "tow" (he can make the "sssss" sound and say "no", but we can't get him to put them together), and any other beginning "sn" sounds also come out as "t" sounds (take for snake, etc.)
"stop" comes out as "top"
"splash" comes out as "plash" or "pash"
"scrape" comes out as "cape"
"scratch" comes out as "catch"
etc.

And as I'm typing this, I'm remembering that he's also struggling with beginning "T blends"...

"tree" comes out as "tee"
"truck" comes out as "tuck"
"throw" comes out as "tow"
etc.

Is this age-appropriate? And are there ways we can/should be working with him to help him with these? People (aside from DH & I) can now understand him about 90+% of the time, but when S or T blends come up, we usually have to translate. And sometimes WE even have to stop and "remember" his mispronunciations to decipher what he's saying.

Any insight you can provide would be greatly appreciated!