Wax man, put the ratchet inside a gallon baggie or equivalent, and remove the two screws the plate will come off reveling a VERY SIMPLE mechanism.
There is a cam attached to the lever, two pawls the latch into the gear depending on the cam position, and the two springs that should be between each pawl and the side of the head.
Clean all the parts, lightly lube the gear and the contact surfaces of the gear hub with a light grease, do the same where the pawls sweep against the back wall and plate and reassemble. Your Dad's ratchet should be ready for many more years of use.