Hi,
As a part of Employee Welfare, you can have a doctor and not this purpose alone.
Secondly, you can issue warning, conduct an enquiry, establish the authenticity of medical reasons, provide him an opportunity. If you think the medical certificates are not genuine, you can still go ahead and issue a warning letter stating, leaves are not acceptable on some pretext. And you can also tell him to look out and the best thing is that you should take his resignation in writing.
For further advise if any, feel free to call me on +91 9900638383