Answer

In the Name of Allah, Most Gracious, Most Merciful. 

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.

In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:

While kissing on the cheek, head or face is allowed between parents and children and close blood relations, kissing on the lips is not desirable or even permissible. While it may be excused between a mother and a daughter, it is not at all permissible between a mother and her grown-up boys as ruled by scholars.

In Islam, we are to avoid things that may inadvertently lead to suspicions or lustful thoughts. One of the rules of jurisprudence states, ‘That which leads to haram is also deemed as haram.’

Furthermore, we are to avoid the doubtful. The Prophet (peace be upon him) said, “whoever shuns the doubtful is better enabled to safeguard his religion and honor, but whoever indulges in the doubtful may slide into that which is forbidden.” (Al-Bukhari)

Almighty Allah knows best.