My son was diagnosed with ASD in 2019 but I was unaware we needed to list this in our tax forms. Can I do that not and going forward or is there a pentaly?

When entering him as a dependent you can select disabled.  However, if the child is under the age 13 it provides no tax benefit to you by doing so.

If the child is older than 12 and disabled you can receive Child and Dependent Care tax credits if you have someone care for him while you work.  And when the child is age 19 or older and disabled you can claim him as a dependent under the Qualifying Child rules instead of the Qualifying Relative rules.

IRS definition of disabled -

Permanently and totally disabled. Your child is permanently and totally disabled if both of the following apply.
• Your child can't engage in any substantial gainful activity because of a physical or mental condition.
• A doctor determines the condition has lasted or can be expected to last continuously for at least a year or can lead to death.

ASD=Autism spectrum disorder?

Because the IRS (tax) definition of disability is based on a person's ability to perform gainful employment, many people with medical definitions of disability are not actually disabled for tax purposes.  The IRS says that if someone can work at least a minimum wage job without substantial special accommodations, that is proof they can do gainful work.  

Even if your child meets this definition of disabled, the only tax benefit for listing a child as disabled is that you could get the dependent care credit if you have to pay for day care after they turn 13, because they can't care for themselves.

There would be no penalty for not listing your child as disabled (if they are), and no need to amend your prior returns.