Does my son qualify for the tax credit as totally disabled if he has autism?

Perhaps, You are permanently and totally disabled if both 1 and 2 below apply.

1. You can’t engage in any substantial gainful activity because of a physical or mental condition.
2. A qualified physician determines that the condition has lasted or can be expected to last continuously for at least a year or can be expected to result in death. See Physician's Statement, later.

Substantial gainful activity.

Substantial gainful activity is the performance of significant duties over a reasonable period of time while working for pay or profit, or in work generally done for pay or profit. Full-time work (or part-time work done at your employer's convenience) in a competitive work situation for at least the minimum wage conclusively shows that you are able to engage in substantial gainful activity.

Note.  

Substantial gainful activity isn't work you do to take care of yourself or your home. It isn't unpaid work on hobbies, institutional therapy or training, school attendance, clubs, social programs, and similar activities. However, the nature of the work you perform may show that you are able to engage in substantial gainful activity.

This is published by the IRS in Instructions for Schedule R (2024) under permanent and total disability