It’s hard to answer this question without knowing the nature of your business, but if his attitude is toxic then it will be affecting the work environment and ultimately the customers. At some point you need to assess whether his value to the company outweighs the damage he is doing to the company. If he is a valuable employee with a skill set that you can’t easily replace and he does a good job at a job that is isolated from others, then a bad attitude is something you need to address but maybe not worry too much about. If it’s going to make or break your business you need to do what’s best for your business.