If there are no symptoms right now then you can postpone the surgery until the pain bothers him or if number of stones increase.
Right now you need to correct his diet to avoid more stones in future
1. Increase fibre intake of his diet by including whole grain cereals and bran, vegetables and fruits with their peel etc.
2. Avoid refined flour (maida) food items like breads, biscuits, cakes etc
3. Avoid saturated fat like ghee, butter, cream, cheese, meat, whole dairy products, ice creams etc
4. Stop oily (reduce oil intake) and fried food
5. Restrict intake of desserts (sweets)
Next Steps
If possible try to figure out the cause of gall stones. I can see your father has fatty
liver. Sometimes liver disease can also give rise to gall stones (stones due to excess bilirubin)
Health Tips
If you decide to not go ahead with the surgery then keep repeating an abdomen ultrasound every 6months.