Weak Memory

Hi all! I have a 4y/o who seems to have trouble with his memory. Often times he will call for me and when I answer him (can be instantaneous) he will say "um, um, um, I forget", or when reviewing his ABC's and working on his letters he will forget letters he easily knew for weeks before without any trouble, or when given a list of choices he seems unable to remember what the choices are even if there are only two or three. It appears that he just draws a blank and will look at me like he is clueless. Is there something I can use to help him? He is underweight, often chilly, quiet though screams and cries dramatically when offended, stubborn, particular, keeps feelings to himself, rarely complains about being ill even when I see he is sick, and likes sweets and hot drinks. He also wants to make up his own rules for life...for instance, he will say "mommy I know this is XYZ, but I am calling it ABC", and he will expect me to know what he is talking about when he calls things what they aren't. I hope that made sense. There is a part of me that wonders if this is all a game he plays to be in control or if there really is a problem that needs to be addressed. He starts school in the fall and I just don't want him to struggle if there is an issue that needs corrected. Any help would be appreciated! Thank you!