I think Reese's is a carnivore, and the reason for his behaviors is due to him starving and having a natural predator instinct.

Lets look at Reese design, he's notably very thin even in his monster form. His ribs and collar bone are on display. Initially I just assumed he was thin. (some people are just naturally thin, its normal)

But look at how his skin stretches, its clearly meant to accommodate that larger form. He has excess skin to grow into. Like he's not meant to be this small.

Some of Reese's symptoms are ones of starvation

feeling tired all the time.

feeling weaker.

getting ill often

During the Twitter Q&A Reese mentions that his favorite foods as a kid was Liver.

From what Dr Kelly said she only started controlling his diet after he went through puberty.

So what is his diet? During the Dinner we see him eating only plain pasta while everyone else has sauce and meatballs with there's. Other then that we know he's allowed to drink juice, ginger ale and eat saltines.

None of these have meat in them.

We can see from these photos of him as a kid he wasn't always this skinny, all the images we have of him post puberty he's incredibly thin. So I don't think it's natural for him to be this skinny.

I think this is directly the result of him having such a controlled diet. I think Dr Kelly cut meat out of his diet at one point and he's a carnivore that needs meat to survive.

On a hot route his paintings bring him a squirrel to eat. He's says he would've picked something else out if he could, but he still eats a whole raw squirrel.

The description of the achievement for him eating Derekson on a Matricide run.

I think the wording of this implies two things

One. Matricide Reese will have the opportunity to eat someone again.

Two. The phrasing of "he's a growing boy" implies he's not fully grown.

I think Reese is a natural predator, he has the instinct to kill not out of malice but in the same way an animal does. The reason his immediate response is eating his mother is due to him being akin to a starving animal.

The lack of meat in Reese diet has helped lead to him being stunted. I think if we let Reese eat more people we'll see him 'grow.'

What that mean I'm not sure. But I needed to ramble somewhere about my 'evidence' that I'm sure is leading somewhere. Probably. I'm not procrastinating on my college coursework you are!