E.T.
When my daughter switched to the 2 year old room, she came home and started sucking her thumb. She hasn't sucked on her hand (she never thumb sucked) since she was a couple months old.
I just asked her why she was doing it and informed her that she doesn't suck her thumb. Fortunately, it was a one day event.
I would correct him and read to him and expose him to correct pronunciations. He will be picking up phrases and annoying habits from his peers throughout his life, you will have to learn to combat it.
Personally, I don't think a speech therapist is necessary. I had a lisp when I was a kid and they didn't even worry about it in school until I was 6 or 7. "Th"ing my "s"s did not destroy me. Your child doesn't even have a problem with lisping, just a bad habit.