Does my 45 yr old autistic son need to file a tax return?

It depends.  If your son is permanently and totally disabled, then possibly.  

When you can claim him as a qualifying child there is not  an income limit, however, it does have a support side.  In order to claim him, you would need to provide over half of his support and he would have to have lived with you for more than 6 months during the year.  Since he had $28,600 in income for 2024, did he use that for his own support and was that over half of his support for the year?  

If HE did NOT provide over half of his support for the year, is permanently and totally disabled and he did live with you more than 6 months, then you would be able to claim him as a dependent. 

I appreciate the reply but because of his age he is no longer a qualifying child, he has been a qualifying relative.  I think the unearned income limit applies, which he is over, doesn’t it?

Q.  Can I still claim him as a dependent?

A. Yes. His  disability means he qualifies as a Qualifying Child dependent. So, it does not matter how much income he has. What matters is how much of his own support he provides (see below). 

Q. Does my 45 yr old autistic son need to file a tax return?

A. Yes. Dependents have a lower filing threshold than regular tax payers. They must file if they have more than $1300 of investment income. He checks the box, on his return, that indicates that he can be claimed as a dependent. He can still be a dependent even if he has to file a tax return. 

There are two types of dependents, "Qualifying Children"(QC) and Other ("Qualifying Relative" in IRS parlance even though they don't have to actually be related). There is no income limit for a QC but there is an age limit, student status, a relationship test and residence test.

A child of a taxpayer can still be a “Qualifying Child” (QC) dependent, regardless of his/her income, if:

1. He is under age 19, or under 24 if a full time student for at least 5 months of the year, or is totally & permanently disabled (at any age). The fact that he is receiving SSDI means he is disabled for tax purposes. 
2. He did not provide more than 1/2 his own support. Scholarships are excluded from the support calculation
3. He lived with the parent (including temporary absences such as away at school) for more than half the year

So, it doesn't matter how much he earned. What matters is how much he spent on support. Money he put into savings does not count as support he spent on himself.

The support value of the home, provided by the parent, is the fair market rental value of the home plus utilities & other expenses divided by the number of occupants.

The IRS has a worksheet that can be used to help with the support calculation. See: http://apps.irs.gov/app/vita/content/globalmedia/teacher/worksheet_for_determining_support_4012.pdf

See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html

Q. When my son files a tax return, will he have to pay tax on his social security?

A. No. 

Social security only becomes taxable when added to sufficient other income. If you are otherwise required to file a tax return, you do need to enter it in TurboTax (TT). TT will determine the taxable portion.

Social security (including SSDI) becomes taxable when your income, including 1/2 your social security, reaches:

Married Filing Jointly(MFJ): $32,000

Single or head of household: $25,000

Married Filing Separately and lived with your spouse at any time during the tax year: $0

One half of his $20, 000 SSDI is $10,000. $10,000 + $8600 = $18,600. That's less than $25,000. So, none of his SSDI is taxable. 

After TurboTax (TT) calculates the taxable portion of SS, it puts the total amount of SS on line 6a of form 1040 and the taxable amount on line 6b ($0 in his case). TT also produces a worksheet  to show how the taxable amount is calculated.