I have a small mixed breed puppy that is 7 months old.the past week he has been very picky about his food.. as of today the only thing I could get him to eat is dehydrated chicken strips.and he drinks lots of water.He poop has also changed to a dark loose stool,he is still very active and playful.I did catch him in the garden chewing on some small green tomatoes.Could this have made him sick

I don't know about green tomatoes. However, I know I've heard that tomato plants (not the ripe fruit itself) can be poisonous to dogs.

It would seem a little odd that a basic tummy ache or issue would last for a full week.

If he's not all better today, I would consider giving your vet a call to find out what they think.