Speak in Full Sentences!!

I don't have trouble getting my students to speak. I've heard horror stories about some students who are so timid and embarassed that they absolutely refuse to speak. Luckily, I've never taught such students. The problem I have with some of my students is that they don't speak in full sentences. I can't tell if they don't want to or if they just keep on forgetting. I used to say "Full sentence, please" or something of that sort in the beginning, but after a couple of failed attempts, I gave up.

I have the most trouble with my fifth-grade student. I think that he doesn't want to speak in full sentences, and I don't want to force him to do something he doesn't want to do. However, other teachers have told me a benefit of speaking in full sentences is that the learners become more aware of grammar. I've told him "One more time, but full sentence." And, I've tried to elicit full sentences by saying the first word. However, it doesn't always work. On the other hand, speaking in full sentences doesn't seem very natural. I know that I don't speak in full sentences all the time. Actually, I don't think I use full sentences when I'm outside of the classroom.

So.. two questions..
Should I push my students to speak in full sentences?
If so, how?!?!?!

[I think this post is a cry for help.. I've been having a hard time eliciting full sentences from my fifth grader.]