If you have not already done so I would encourage you to have an open and honest discussion to find out the root cause of the problem. If you show a sincere desire to understand and correct the issue you will probably get a positive response from the Soldier. You do not have to do this but it shows you care.
There are several methods to correct a negative attitude. In this case I might suggest the following: Have the Soldier go to the post library or public library and check out a book called Over the Top by Zig Ziglar.