It is a difficult one because he can sue for unfair dismissal.
If he is off sick, even though he is out of leave, if it is for less than 3 months it is deamed temporary and he can expect his job top be there when he comes back.
If more than 3 months then you may have rights.
The following link explains the situation better
You need to get a medical report from his doctor an whether he will be able to return to work.
It is a difficult situation for you.
The next link explains what you need to look at when deciding to terminate him.
You have to do your best to assist him before you terminate.
Is there anything else I can assist you with?