These questions are being answered by Learn HR, a market leader in the provision of HR and payroll training and nationally-recognised professional qualifications.
Yes, you have an overriding responsibility to ensure that the employee is healthy enough to work and can do so without endangering himself or others. But before doing anything else, consider whether the employee might be given light or other duties for a period to enable him to recover gradually. Perhaps that is all he needs.
If that is not practicable you should suspend the employee with appropriate sick pay and write to his doctor explaining the nature of the work the employee does, what it involves, and why you are concerned. Quite possibly the employee told the doctor that he feels fit enough and the doctor had no serious doubts, not realising the demanding nature of the work and perhaps the employee’s wish not to lose wages.