Why would Allah SWT say this? 
You are to Me like My Unity and my Uniqueness. The time is approaching when you will be helped and will be known among people. You are to Me like My Throne. You are to Me like My son. You have a standing with Me of which the people have no knowledge. We are your guardian in this world and in the hereafter. When you are angry, I am angry; and when you love, I love.
Haqeeqatul Wahi pp. 70-108
Recall what Allah has said in one of the most inspiring and majestic chapters of the Noble Qur'an:
Say: He is Allah, the One and Only;
Allah, the Eternal, Absolute;
He begetteth not, nor is He begotten;
And there is none like unto Him.
The Noble Qur'an, Sura Al-Ikhlas
Mirza Sahib claims that Allah SWT said to him "You are to Me like My son". Would the Allah SWT who said in the Noble Qur'an "He begetteth not" really say such a thing? Would Allah SWT use such a metaphor when He has been so categoric in denying any progeny? Allah SWT, in the Final Revelation, revealed to the Final Prophet SAW, rebuked the Christians for daring to suggest such a thing. Why would the same Allah SWT contradict Himself?