Are you talking here about your form control or the form control that's set for a particular field type?
I created a new field of Boolean type and it's form control is automatically set to "Checkbox". This means that It will show me a checkbox type control for this particular field while trying to enter data for a custom table.
How are you generating this dropdown from a custom table? Are you using a custom table repeater? If yes, you could fetch data only for those records who value is "Yes" by setting it through the where clause of the repeater.
Let me know if this helps. I am attaching a screen shot for your reference
Custom table field reference