Hi K.,
Since your child is sensitive already, with allergies, you might want to look into something called Sensory Processing Disorder. The books "The Out of Sync Child" by Carol Stock Kranowitz and "Sensational Kids: Hope and Help for Children with Sensory Processing Disorder (SPD)" by Dr. Lucy Jane Miller offer some great insight into this disorder. (I personally like Dr. Miller's book better, as I feel it's more user friendly.)
SPD is a neurological disorder. Your son could have sensory over-responsitivity, which can manifest in one or more sensory systems. Some of the ones that can be affected include sound (auditory), touch (tactile), taste (gustatory), and smell (olfactory).
We knew since my daughter was born that she was different. At the age of two, she grew from wearing a size 2T to a 6x. My daughter has trouble with needing to constantly have her mouth stimulated--she'd rather eat, even when she's not hungry. Like your son, my daughter liked to drink Yo-Jo and juice...but she drank so much, her Pediatrician asked me to get her switched over to water, because the extra calories weren't good for her. That was a challenge and a struggle.
She also has trouble with an over active bladder and constipation. She has trouble processing information or making a decision rapidly, retaining information, and needs time to shift gears when going from doing one thing to another. Unlike children who love to run around and play, my daughter would much rather sit and watch TV or a movie/video. Since the age of two she could sit and watch the same movie over and over until she had it memorized. Because only when she had it memorized, did she not get scared or upset by what was going on on the scene; she had "control" over the situation.
She gets upset and cries very easily--at least, she no longer does the "fight or flight" response. That used to be when she'd take off running to hide somewhere, screaming or crying at the top of her lungs.
For your son, if he has SPD, SPD can affect the taste and texture of the food, how it feels and tastes in his mouth. He might only like hard, crunchy foods. Or soft, squishy foods. Some kids only like pasta; others hate it. My daughter has a very limited variety of foods she'll eat, and at the age of 8 yrs, we're trying to coax her to try other things.
A child with SPD can be either overstimulated or understimulated. The books do a good job of explaining all of the different areas.
It wasn't until I read these books that I diagnosed my child. When I asked the Childhood Specialist we finally took her to see when she was 5, she told us that yes, my daughter does indeed have SPD. It falls under the Autism Spectrum, and isn't recognized by the psychiatric association, which can make it extremely difficult to obtain any medical or behaviorial help. Lucky for us, my daughter is able to get speech, OT/PT, and Reading Plus help through our school district. However, the Children Wellness & Fitness clinic I take her to to help her with her exercising and weight would only be possible if we had our current health insurance.
My daughter is also very sensitive. If your son is, too, another great book to read is "The Highly Sensitive Child" by Elaine N. Aron, Ph.D. A helpful website might be The KID Foundation, which was set up by Dr. Miller to help publicize SPD and for further scientific research and study.
I hope some of this is helpful.
A.