An obese kid who suffers from junk food addiction is put on a healthy diet. As he struggles through withdrawl, he starts to hallucinate that his happy meal toys collection is coming to life, and he embarks in an adventure to save the loved one of Grimex, the most popular of his toys, from his neighbor, a bully kid who works as a delivery boy.

Hey mate - first off, this logline is too wordy. You need to trim it down.

Secondly, and more worryingly, it's hard to sympathize with your protagonist. I think the general consensus among most audiences is that childhood obesity is a pretty bad thing. The stakes for a child hallucinating and going about his day under delusional influence are so much higher than whether or not he can rescue his toys that I don't think an audience could settle into the adventure you're suggesting presenting to us.

How can I focus on the rescue of a toy, when I know that your protagonist's mental state is seriously compromised because of a very real epidemic facing children all over the western world, and that what he is seeing is not real, and therefore he could be putting himself in VERY REAL danger?

I actually don't know what I'd tinker with you make this story work, but I think you have a problem with your setup being way more serious than the tone and stakes of the rest of your logline suggests.

i think you can make this more concise. Eg: An obese boy is put on a strict health food diet and as he struggles with withdrawal from junk food he starts to hallucinate etc...

First off your logline is way too long. Remember that the logline should impress the heart of the story, the what's at stake, onto the reader/agent who you hope will read your full screenplay. Loglines should be 25 words or less, but no more than 30 words.