Q: My student can do R in all words and in all positions, but he prolongs it. Do I need to teach him NOT to do this?
I would teach it to him if it did not go away by itself within a reasonable period of time. I am not sure what that reasonable period of time is, but I would be willing to give him 6 months to a year to straighten this out.
I probably would give him a few lessons on this and then let him go with periodic checks every few months. Use negative practice in these lessons, and have him compare long and short productions. Make a long one, and then a one short, and go back and forth between the two. This is ear training. Teach him a simple rule, “We make R short like all other sounds.”