Hi Luke,
This could be that the motor bearings need cleaning and re-greasing , the gearbox is dried out, or even the belt. It has been known that the brushes can cause squeaking. Whichever the case I would not recommend that you do the work yourself, do it together with your Dad.
Stripping the motor and gearbox is not hard, but tricky. Directions on how to strip the motor can be found in the 'Unofficial Service Manual', this can be found on the home page under 'downloads'.