Hi, thank you for your query, I will try and assist you as much as I can.
Essentially, you want to bring your son, who is abroad with your mother. The concern is that you did not mention in the application for your spouse that you have a child.
Your option is to apply for indefinite leave to enter for your child. Here is the from the Home Office website: https://www.gov.uk/government/publications/children-set07/children-set07#set71-what-are-the-settlement-provisions-for-children-under-the-rules
If you make this application, please bear in mind that the Embassy may request a DNA result, because your spouse did not mention your son in the initial application. Besides, and I cannot advise specifically on this forum, the Embassy could raise an issue of deception.
It is important if you do make this application that you cover the above issues.
I hope I have dealt with the question you asked.
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