Possible to get a visa for being father of Thai child as before ? WITHOUT legimitazion of the child, I mean.

even if yes for a single, possible also to get a multi "O" ?

You may be able to obtain a single entry Non O visa on the basis of being a child's father .

Doubt a multi-entry visa or an extension of stay will be available without evidence legimitimisation .

Edited June 17, 201511 yr by nzexpat

It would be easy to obtain a single entry non-o visa based upon being the parent of a Thai with just your name on the birth certificate.

There are few embassies and consulate that will issue a multiple entry non-o visa without being the legal father of your child. Penang is the only nearby location that will do it but you will need to show financial proof. Having 200k to 400k baht in the bank is the numbers often mentioned in reports of getting one.