While having someone off work can be an inconvenience the important issue here is to ensure that your employee does not suffer permanent ill health. If he has to stop working both the he and the company will suffer.
Find out if the absence is related to his exposure to the product being used to spray the wood. If it is classified as a hazardous substance under the COSHH regulations. You should have a risk assessment. I would consider referral to occupational health to ascertain if his ‘sore chest’ is related to this exposure