Hello and thank you for your question.
What a great person you are for taking in this fur baby.
I have seen many dogs do this I myself have 2 that will only eat their kibble 1 piece at a time and they too put it on the floor.
Some behaviorists will tell you that this stems from the dogs natural instinct to remove their food from the pack to prevent others from getting it. It may your boy learned this behavior on the street where he likely had to eat from trash cans or anywhere he could. His instinct would tell him to grab the food and walk away before either he was caught or another stray could come and take it away.
I would not worry as many dogs do this and it just may be his way of feeling secure that his food is safe.
I hope I have put your mind as ease that this is normal behavior for some dogs so there is nothing wrong with him. Like humans pets sometimes too have there little quirks.
Please feel free to reply back if I can be of further assistance.
Candy
Veterinary Technician
Practicing Veterinary Technician for 23 years. Veterinary Practice Owner, Technician Trainer