T.,
My 6 yo started talking at 9 mo like a normal baby, saying mama, dada, baba for bye bye, etc. Then around 12 months he just quit. He is special needs anyway, so when he quit talking I immediately got him into ST. The speech therapist worked with his acceptance of textures and touch in his facial area, but still couldn't get him to start making sounds again. I had intended to do baby sign with him but found myself overwhelmed by his special needs when he was born so hadn't followed through. Around 14 months we really started on sign language. By age 3- when he just started making sounds and talking again- he had over 100 signs in his vocabulary. We continued in speech therapy until last year, when he graduated out. He still stutters every now and then but his words are very clear and his language structure is way over that of the average 6 yo.
Even though your son isn't talking yet, there is a lot going on inside him cognitively. Learning sign doesn't prevent or delay the development of verbal language skills, learning sign enhances language skills overall. Possibly giving him a way to communicate with you that is non-verbal, through sign, would help him through the frustrating toddler years. It certainly helped us! When my son did start making sounds again, he was right back where he was when he started, with not very many consonants. Everything was da duh di all D's. As his vocabulary expanded, it became very difficult to understand him. But if he knew the sign for what he wanted, it cleared up the frustration pretty quick! He even remembers his younger brothers birth, he was only 2 at the time but I attribute his memory to is ability to verbalize- he signed "mama, hurt, daddy, help, baby" over and over for weeks after the birth.
Signing doesn't have to take up a lot of your time. We bought a paperback ASL dictionary and used "Signing Time" a little. I made a goal of learning one sign a day, it ended up being more like 2 a week that would actually stick with us. There are also free dictionaries on the web. I still use sign with my older kids- wait and no and stop are particularly useful. :o)
While having him around other kids might stimulate him to talk more, it might also push him to withdraw further, if he feels intimidated by their ability to communicate with each other, and doesn't make a friend right away. Is he an introvert or an extrovert? I'd keep a close eye on his emotional reactions to a MDO program at home, and make sure he isn't regressing in any other areas. I would be concerned about problems with other kids and the staff, and him not being able to communicate them to me.
There is nothing wrong with your parenting that he isn't talking yet. Lots of very smart people didn't start talking until later in life. There is still plenty of time for him to develop his sounds and form a vocabulary. He's working on it all the time, just listening to you and those around him. You are doing a great job with him and you do not need to be losing sleep!!
Having TEIS to help with speech therapy is great. I know you are close to aging out- I personally would recommend against ST through the school system, it was a total waste of time and energy for us, and it was not nearly as good as the therapy we received through Le Bonheur. If you have insurance that will cover ST for him, look for a program through UT (not sure if Knoxville has a children's hospital.) I'm sure TEIS will help you with the transition.
Good luck!!