The mother of my child has sole child custody.
She has informed me she is going to give guardianship of our son to her brother.
Please note that I am not licensed in your state so I am unable to provide any insight into your state's child custody laws.
To better answer your question, I would need to know whether the sole child custody that you refer to is sole legal and sole physical, or joint legal and sole physical.
Assuming you mean joint legal and sole physical, meaning you have equal ability with the mother of your child to make legal decisions regarding your son, but she is the primary physical custodian of your son, then you should certainly oppose her desire to transfer guardianship to her brother.
If she can’t maintain custody and care of your son, then it stands to reason that you should be the one maintaining custody and care of your son. You should file a motion with the court objecting the guardianship and requesting sole physical custody.
If for some reason a guardianship were put in place child support would be calculated according to the mother’s income. The child support that you are paying to the mother would need to be directed to the guardian through the Friend of the Court.