The way you explained it here should be perfectly ok for your subordinate as well during a one-on-one meeting.
However, there are two other things worth taking into account:
Before promising a raise you need to be absolutely sure that the raise can actually be accomplished. Otherwise you'll risk to break your subordinate's motivation.
Your subordinate might actually love his job at restaurant. If that is the case, the extra money he earns there isn't really the important factor. So be prepared that he'll refuse to leave his second job for this very reason. In this case you could consider explaining the benefits that the higher grade can have. Focus on the new and more interesting aspects/perspectives that a jump in grade could open to him rather than the higher pay.
( 7 months ago )
A junior member of my (small - 4 staff members) department has been with us full-time for almost three years. However, he has also kept a job working in a restaurant for three nights a week, and I'm concerned that it is affecting his ability to advance.
As his line manager, I'm happy with his work in general - he gets work done, and is at work for the 37.5 hours a week he is required to be. However, there are times when he is obviously tired, and while his work is solid, he never seems to push beyond the minimum effort (other staff will put in extra time as required and then recover it as TOIL later) - mostly because he has to go to this other job.
At the moment, I am not able to point at his contributions and make a case with my managers that he deserves anything more than a standard increase - whereas a jump in grade would easily cover the money he's getting from the other job.
We have nothing in our contracts that prohibit second jobs - and I'm aware that I'm not in a position to force him to quit the other job.
How should I approach him with advice that by working the second job, he is actually hurting his chances of promotion and more money within his primary career?