Under the circumstances existing currently, the ADA has specifically allowed employers to bar an employee from physical presence in the workplace if he refuses to have his temperature taken, refuses to answer questions about whether he has COVID-19, has symptoms associated with COVID-19, or has been tested for COVID-19 and is positive.
If the employee claims under the ADA that they have a medical reason for refusing a temperature check, the employer can still bar them from entering the workplace. However, their reason should be documented and accommodation should be provided to the employee (i.e. work from home). If accommodations of any sort are not possible, then the employee will be on unpaid leave. They can also be fired if the role cannot stay unfilled.