How is Form I-824 used to protect an EB-5 beneficiary from retrogression?

The priority date is determined upon the filing of the I-526 petition. As long as the I-526 was filed before the child turned 21, he should be protected.

Your son's age was frozen at 20 years and 2 months old before the I-526 was approved (i.e. 10/1/2015). Since your priority date is May 2014, you can now file the DS-260 for yourself and all accompanying family members, and you should do so as soon as possible.

Your child should come under the Child Protected Status Act (CPSA) instituted by President Bush. However, I would seek the advice of an immigration attorney to review your application and approval documents just to be sure.

You do not need an I-824. As long as the I-526 was approved before your son turned 21, he will be OK.

If the priority date is current, then just file the DS-260. If it is not current, then the I-824 can be considered.

Please note that the Child Status Protection Act allows you to deduct the amount of time that an immigrant petition (I-526) has been pending, which in your case seems to be about 16-17 months. This means your child's age was frozen at the time of your I-526 filing at 20 years 2 months old. According to the November 2015 Visa Bulletin, the priority date for being able to start the visa application process (DS-260 filing) is May 1, 2015, which makes you eligible to start that process. You have up to one year from the eligibility date to file the DS-260 and start visa processing. I-824 does not help you.

Pay the fee bill and file the DS-260 for him; if not issued, file the DS-230. If you are in the United States and able to file the I-485, that is best.