my son is now 12 years old and by the time he was 6 or 7 his tongue was pretty much normal. it does still protrude when he is very tired or has a cold but other than that it stays in his mouth. i've never heard about any surgeries for this though. his doctors told us to give him things to suck on when he was little and that seemed to work. he did have a speech therapist as a baby that told us to drip hot sauce on his tongue but i wouldn't recommend this.
I adopted three babies with Down Syndrome. They are all adults now. Protruding tongues were a problem we worked on a lot. It was a behavior that they all overcame. My daughter who had the most problem with it, took the longest to learn. It was all about me being consistent, and flicking her tongue back in with my fingers, saying "tongue in" to her, so she'd learn to be aware that it was out. There was one little girl in her class who had such a problem, that she had to wear a stretchy cloth band, like a hair band, going around her chin, and up over her head, to hold her mouth shut. It worked, but it took a very long time.
Surgery for tongue protrusion is quite an extreme approach. It used to be recommended when the child could only breathe with his or her mouth open - in these cases the tongue was so large that it blocked the air passage when the mouth was closed. About 30 years ago it was sometimes used to make a child look less "Down Syndrome"-y! Typically a triangular cut (base of the triangle being actually the tip of the tongue - tip of the triangle ending in the middle of the tongue) was made to remove a "chunk" of the tongue. The sides of the triangle are then sewn together. THE TIP OF THE TONGUE HAS THE MOST/FINEST MUSCLE CONTROL SO BY REMOVING THE TIP, SPEECH CAN OFTEN BECOME UNINTELLIGIBLE! This may not be as much of a concern if your child is nonverbal but otherwise, the surgery will make your child difficult/more difficult to understand. I wouldn't consider the surgery unless breathing is an huge issue.