The standard deduction is listed on form 1040 line 12. It will just be the amount $27,400 (for married filing jointly when both spouses are 65 or older), it isn't calculated anywhere else on the form.
Your disabled dependent will be a $500 credit on form 1040 line 19.
In your original question when you asked about "$28,300" for "3", you might be thinking of table 7 in publication 501. But that only applies to you and your spouse. If you and your spouse check 3 of the boxes, the standard deduction would be $28,300. But the boxes are,
- spouse 1 is over 65;
- spouse 2 is over 65,
- spouse 1 is blind,
- spouse 2 is blind.
These bumps to the standard deduction never apply to other family members, even if you take care of them and claim them as dependents. If you claim your son as a tax dependent, there will be a credit on line 19, not a bump in the standard deduction.
