I am legally married to a Thai in Thailand. Have 1-year retirement visa. Our son is almost 3 years old, born in Thailand. Now he has received his German passport. We want to leave on a holiday to Europe but do not need a visa for him as he has a German passport. What is required from Thai immigration before we can leave with the child. He has no stamps in his passport. Please can someone advise. Thank you.

The childs mother is Thai, that means the child has Thai nationality. Just apply for a Thai passport for the child. Upon exiting present the Thai passport and at checkin at the airport both passport. When you come back present the Thai passport again, as that shows he doesn't need a visa.

For passport application: http://www.mfa.go.th/web/473.php?id=3167

There are several locations where you can apply.