Sign language is a wonderful tool for helping young children communicate their needs. I remember using both verbal and sign language to communicate my daily activities to my daughter and being so excited when she started using it on her own to communicate with me. Keep it up - before long he"ll likely be communicating with you in the ways you model communicating to him.
Let's talk about developmentally appropriate behaviors. It may be a relief to know that kids throw, spill, taste, swirl, flick and otherwise "play" with food and other objects as a means to understanding their world. Think of it as an experiment in science, physics, art, psychology, etc. all wrapped up in a single meal (and maybe in every meal and snack from birth to age 4)! Through sensory exploration children begin to learn the laws of physics (there goes the water out of the cup), emotional intelligence (mom sure looked over here quickly when I did that) and color hues (oh! green puree plus orange puree makes brown puree) simply from exploring their food. So, while I'm not saying you can't guide him toward more socially acceptable behaviors, it may help to know that these behaviors are a normal part of your child's learning process and not meant to be a direct assault against your sense of cleanliness or manners.
So, how do we guide them toward more appropriate behaviors? First, children need to be able to explore their environment in sensory-stimulating ways in order to get their need for stimulating learning experiences met (check out Play At Home Mom for more ideas on fun and creative sensory exploration). Once children have a regular outlet for sensory exploration they then become free to learn other uses for food (as adults we call this use of food "nourishment" and we use "manners" - sometimes I laugh at our ability to take the fun out of everything). This isn't to say your son won't still explore his food at the table, but he may be more likely to eat his food and perform more sensory explorations at other times - just keep in mind he's still going to eat in developmentally appropriate ways, which aren't as neat as most adults eat! LOL
Once you're meeting his need to learn about his environment you can guide him toward the socially appropriate behaviors at the table. So when he throws his food off the table you can say in a kind tone, "you want to throw the peas, but peas are for eating. You can eat the food with your fingers or use a spoon" or "I know it's fun to throw the carrots, but carrots are for eating. When you're done we can go throw a ball (or fill in your own activity)." He'll need a number of gentle and consistent reminders throughout the course of the meal, and for several weeks...even months. Expect him to play with his food; encourage him to keep it on the table. I know a lot of parents embrace the throwing and then have their child help clean up. This is a great option because he is learning cause and effect while also having some time to connect with you and help, which builds a sense of capability.
This brings us to the the third part of your dilemma, so let's talk about behaviors that are driven by unmet needs. Rudolph Dreikers says, "there's no such thing as a misbehaving child, only a discouraged child." Together, with Alfred Adler, he often discussed the meaning behind children's behaviors, which they called "Mistaken Goals of Misbehavior." Jane Nelson and Lynn Lott developed an approach to these behaviors called "Positive Discipline," in which they help parents identify the 4 Mistaken Goal of Misbehavior." Once parents understand the goal of their children's misbehavior they are then able to help children learn appropriate behaviors by first meeting their children's unmet needs, then helping them learn to meet their needs in more socially appropriate ways.
My guess is that he is either screaming to get your attention, screaming because he is excited about the science experiment of eating, or it's possible he's screaming because he really hates the high chair and would be more content in your lap. We'll address the former because the latter will take care of itself when he has a chance to do those science experiments in other settings, as we discussed earlier. So here are some things you can try to help guide your child's behavior. When he screams you can offer him your company or give him some alternative ways to get your attention:
- "Oh, you'd like to get out of your highchair and sit on my lap. You can say "lap." Then let him sit on your lap and eat.
- "Oh, you'd like for me to sit with you and eat," then join him.
- "You'd like my attention. You can get it by calling my name "mama." Then join him at the table.
I know my daughter loves to sit and eat as a family. In fact, when I feed her by herself she is less likely to participate in the meal in ways that meet MY needs (sit quietly, eat neatly, finish you food). So I try to remember her needs when she spills her water, drops her strawberries and scrambles down to play tag with the dog.
Discover the meaning behind your child's behavior, the need that is driving them to scream, cry, whine, hit, run, etc. Once you understand your child's needs you will be more equipped to address the behavior and begin to guide them toward expressing those needs in positive ways instead.