I'm not sure that there's anything to handle. If his doctor has declared him unfit to work, then there is no possibility of him going back to work regardless of what the individual thinks himself. So there is very little the employee can do about it - I assume that it is the employee who is saying he is going back to work, not the employer?
I came across something similar in an organisation I worked for. Employee had heart bypass surgery. Shortly afterwards, this employee, who was one of those people who lived for his job and was lost without it, wanted to come back to work. But, in the end of the day, he was not passed fit by his doctor, so there was no possibility of him returning in spite of his proclaimed intentions. He had to wait for approx. 12 months until he was recovered and his doctor certified that he was fit to work again.