Mothers responsiblity for 25 year old son.

I am married to a Thai woman and she has a 25 year old son from previous. I am a foreigner. The son got his girlfriend pregnant and they don't want the child. We have said we will pay for all procedures should they want an abortion. Yet her parents want a compensation payment to them. Under what law can they demand a payment? At what age in Thailand is the mother no longer legally responsible for her sons actions/decisions? Thanks & Regards.