Why the employee was selected with "short interview"? Now for short interview, there is short employment and for the short employment, you want process termination also to be short. Do companies grow with these shortcuts?
Rather than issuing a letter of termination to the employee, call him to the HR office, explain him about shortfall in his performance even after the training. Since he cannot render the desired level of performance, let him submit letter of resignation. Follow a normal process of separation.
Notwithstanding his "short" tenure in your company, pay him the wages for the period that he worked in your company.
I suggest to have a robust interview process instead of Short interview. I opine that the interviewer is only responsible for this shortfall. If the candidate is an experienced person he might have left his previous employment and joined you with lot of expectations and enthusiasm and in case you discontinue him with in a short span it will be difficult for him to get a job immediately.
Considering all the above facts suggest you to call him, have a discussion, give him some time, organize some more training and if he is not meeting your requirements even after extended training then you can take a call. However suggest you to strengthen your interview process otherwise the organization's image is going to be damaged and difficult for you to get the candidates in future.