Why is the set amount for dependency only $4150? The amount should be $12000 since he is my child. Usually the set amount of $4150 is for dependants who are relatives.

You sound confused by the amounts that are being used in the preparation of your tax return.

You do not get a personal exemption for a dependent any more.  The new tax law eliminated that.  The amount of $4150 comes into play to determine whether someone who is not your qualified child can be claimed as a dependent. If the "qualified relative" you want to claim as a dependent has income over $4150, they cannot be claimed as a dependent.

The $12,000 is the standard deduction for someone who filing Single or Married Filing Separately.  The standard deduction lowers the amount of income you are taxed on.  It is not a refund.

https://ttlc.intuit.com/questions/4482394-how-will-tax-reform-affect-my-2018-federal-tax-return