He's still pretty young. He understands what you want, he just doesn't want to do it. He probably has a reason, even if you never figure out what it is. As frustrating as it is, I would encourage signing (just so he has a way to communicate with everyone, not as a means to speech), and not pressure him *at all* to use speech. Give him a few months to just be a baby. There is so much going on developmentally at this age, and so much pressure to speak, to potty train, wean from nursing/bottle/paci, sleep through the night, etc. I would let him be a baby in this area for a little longer.
In a few months, I would start talking to him about using words, and how that helps you to understand him better, and how you are excited to hear what he has to say to you, etc. Let him know you can wait patiently until he is ready to speak, and that it's ok if what he says doesn't sound "right"; he will figure it out soon. Sometimes, if you just back off and make it a non-issue, they have the room to decide to do exactly what you wanted them to do in the first place, but it becomes *their* choice. They don't feel like you are forcing them into it. (haha, this has been the case with all of our children, who aren't at all strong-willed, nope not at all, not one bit, except when you want them to do something, hehe!)