I know this post was a long time ago but this reminds me of me! My daughter went through the same thing...her words decreased then out of no where in December she just started talking. I kept working with her. She understood and was retaining everything all along. She just wasnt ready. She just turned six. I just finished reading with her...not full books yet but she reads full sentences by herself. And she loves music...a big thing that I believe helped was singing her fav songs and leaving out an easy word or two and having her fill them in. Before I knew it she was singing with me, then singing by herself. Your son will get there! Keep talking to him and try singing and dancing to his fav songs!

I also have a non-verbal son, he is 10. He used to point or take my hand and lead me to let me know what he wanted or needed. He is still non-verbal, however I believe he will get there because he has come such a long way. He started sign language at 3 and knows over 200 signs now. He used the Picture Exchange system for a while when he was younger also. Now he uses the LAMP app on an iPad as well as signs. He also tries to talk more than he ever did and will write down what he is saying if you can't understand him. He even pulls up Google on his iPad and will type things in the search box. He loves reading and knows what the words are he just can't say them all. So just keep working and don't give up your child will get there!