No result in the Thai's Law.
You can not fire him out of work because when he applied to take a leave,you did not fire him off and let him to take a heart surgery.
If you did not let him to take a heart surgery then and fire him off the work, you have a right to choose whether to take him back is work or fire him off you may Change his position from physical task to office worker (white collar worker). you may consult about the wages. When he mishandle the joie you offer or he start feeling unwell while working you may consider about the result for the both sides.