Explain that he can't take decongestants and that you cannot sell him this product.

In theory you can't sell him the decongestants but you will need to consult the pharmacist and deal with the matter more delicately

Explain to the customer that you are just going to check with the pharmacist that the product is suitable with the presribed medication he already takes. 

We would always discuss an issue like this with the pharmacist as there is a potential interaction between the 2 medicines. It is also good for customer service if it is done correctly.

Sell him the product as he should have read the label.

This is a really poor attitude to have and the customer is not being put first in this situation. It also contravenes the Medicine Sales Protocol