Good idea r.e gearing down, propose that to see if it reassures the loved ones. It may not slow him down much but it is a good marketing approach to sell the idea so that your father can recover a better quality of life.
A 55 or 60 tooth rear sprocket does give great confidence and stall free agility at rediculously slow speeds. Add a Recluse clutch if possible if it seems advisable depending on father's condition.
Lots of safety gear and possibly accompanying your father on a ride or two might help, especially if rest of family can see him enjoying himself and in control of his bike and his destiny once again. Pride is an important component in quality of life.
Only you as a fellow rider and devoted son can truly judge the wisdom of him riding again.