Having a disabled dependent doesn't give you a direct tax credit.
If your son is unable to care for himself and you have to pay for day care so you can work (this would include if he is a danger to himself if left alone) you can claim the credit for child and dependent care expenses even though he is older than the usual age cutoff.
When your son turns 19, and is out of school, he would normally stop being a dependent. If his disability prevents him from working, you would still be able to claim him as a dependent even after he turns 19, if he meets other tests (you support him, he lives at home, etc.).
There is a box on his personal information page you can check to indicate he is disabled, then you can enter your other expenses and deductions as usual, and Turbotax will include anything that is allowed.
