Thank you for checking in, dorsetduckowner.
I bought him on May 30th along with 2 smaller females and a pair of unidentified ducklings (which I also posted for ID help on the general forum) from a poultry shop in Jatujak pet market. I guess he was almost 3 weeks old at that time. Maybe I am wrong but he must be at least 2 weeks at that time because the smaller females should be at least one week.
Before I learn about proper diets for the duck last week, I have fed them a variety of foods including chicken's food, rabbit's food, dry dog's food, canned cat food, fish ball, vegetables, fruits, my leftover foods. And for this bigger fella, I also used to give him some dessert. Now I am feeding them rabbit's food and chicken's food with boiled vegetables.
I will take more photo and measure their weight tonight for more information.