Permits and visas are typically required when entering a country, not when leaving. The main reason they might want it in this case is to check overstay and/or trafficking of a minor. In your case your son is a minor, you are traveling with him. Minors traveling with their parents cannot be charged with overstay.
https://www.gov.uk/permission-take-child-abroad
Make sure you have permission from his mother (if you’re the father).
Credit:stackoverflow.com‘