I work for a small company and recently hired a new employee when one of our senior employees resigned. He wasn't the ideal candidate for the job but we ended up hiring anyways since we were really struggling to cover the duties of the resigned employee and in a desperate situation to find a replacement.
Now that he is hired, he is a very friendly and easy-going person but clearly has no clue about how to make conversations at a professional level with clients, boss, or coworkers. He literally speaks his mind and just says whatever comes in his mouth regardless of the environment or situation, thinking it's funny.
For instance, on one occasion he told our clients, "if we get paid some extra money maybe we will think about supporting the new project" or something along those lines as a joke which is clearly not how you talk to or negotiate with clients. And clearly it's not his job to talk about money with our clients anyways. He is just a new hire.
Now we are planning to hire a second person to replace another senior employee who will be retiring soon. It's a complicated scenario since the senior employee is not physically fit to perform his duties due to old age and going through a tough situation with the idea of retiring. Then this new guy sits here and makes a joke about it saying "unfortunately we don't have a choice but kick you out" again as if it's funny. I don't want to lose this new guy as well, so I am not sure how to address him about his behavior, especially his awkward and unprofessional comments which he thinks are funny.
He even made similar comments to the boss stating "he really appreciate 10% raise in salary for a job well done", when the boss congratulated him on his hard work, indirectly hinting he wants more money and not just mere appreciation. This incident happened during a weekly meeting with other employees present.
Being a small company, we can't afford losing employees, but this kind of immature and unprofessional behavior is not appreciated either.