If my son only received disability and got a FORM SSA-1099, does he have to file taxes?

No.  If he has no other income and his social security is not more than $50,000 and he is not married filing separately, then he would not need to file a return as his social security would not be taxable. 

If he is married filing separately and 85% of  his social security is more than $14,600, then yes, he would need to file a return as he would have taxable income over his standard deduction. 

If you fall into the following, 85% of your social security is taxable income

• Single with income above $34,000
• Married Filing Jointly with income above $44,000
• Married Filing Separate regardless of income

If you fall into the following, 50% of your social security is taxable income

• Single with combined income between $25,000-$34,000
• Married Filing Jointly with combined income between $32,000 and $44,000

Your combined income is calculated by adding your

• AGI plus
• Nontaxable Interest plus
• 1/2 of your social security Benefits

Social Security Benefits Taxes