The current law was in effect before 2020 so is the same for 2020 and 2021. As noted by @ColeenD3, your son does not need to be disabled for you to claim him as an "Other dependent." He just needs to meet these tests:
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The person can't be your qualifying child or the qualifying child of any other taxpayer.
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The person either (a) must be related to you in one of the ways listed under Relatives who don't have to live with you , or (b) must live with you all year as a member of your household (and your relationship must not violate local law).
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The person's gross income for the year must be less than $4,300.
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You must provide more than half of the person's total support for the year.
If he is your "other dependent" (and was in 2020 as well by meeting the same tests) and lived with you for more than 1/2 the year then you would file Head of Household in 2021 and amend your 2020 return to file Head of Household as well.
Here is IRS Publication 501 for reference. See table 4 for the rules for a qualifying person for filing head of household and table 5 for the dependent rules.
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