In some cases, the son might be forced to act because of the impact on other family members, like younger siblings. He sees the mom's drunkenness affecting them negatively, so he takes matters into his own hands. It could also be that the son has reached a breaking point after seeing his mom's self - destruction over time. He wants to save her from further harm, whether it's physical, mental, or emotional harm that the alcohol is causing.
One reason could be concern for her health. The son might see the negative effects of alcohol on his mom and feel the need to force her to stop drinking. Another reason could be for the family's reputation. If the mom's drunken behavior is causing problems in the community or among relatives, the son may try to make her change.