Hi B.,
My son began that after his 15 month vaccines, which I later linked to his sensory and auditory processing difficulties.
You may want to read about auditory processing and understand how to work with him. The brain cannot interpret and respond immediately even though the voice is going in. You may need to give him an additional minute to process before adding any other info. It may be that you look directly at him when speaking and wait for response (beyond huh?) before adding. There are lots of techniques for helping like using the main word first. Also, you may see him doing something like this: Dan - would you like chocolate or vanilla, he answers Vanilla only because that is the word he is processing. You give him vanilla and he cries he wanted chocolate. Sometimes, this is a symptom as well, so be in observation when you offer A/B choices.
BTW - both of these are treatable and if you are ready, I would be glad to discuss with you more. For now, you need to read and understand so you can evaluatle him. He is too young for any formal eval, so to do early intervention, you have to really be his support.
Thankful Mom with fully recovered, blessed son - J.