Each child is different. AND then at each age, they go through food "phases." ALL THROUGHOUT childhood, they go through this. My friend's Teenage child is going through yet ANOTHER food phase of likes and dislikes.
For me, my daughter eats anything, since a baby and has a good hearty appetite... she will sit and eat in one sitting, then also graze on things. My son on the other hand, is SUPER picky... he will not just sit and eat all in one sitting. He is a STRICT "grazer." Meaning, he will pick and eat little by little throughout the day... but only when HE is hungry... not just because we say it's meal time.
What your son is going through it normal. Don't compare him to the other child. Find what makes your son tic, and then feed him accordingly. Just do not turn "mealtime" into a battle or "punishment" thing... otherwise, what should be a nice pleasant meal with the family... with just turn them off. And then they will associate "eating" with pressure and discomfort.
Don't worry, your son won't starve. But, ask your Pediatrician about other types of vitamins. My daughter loves the "Animal Parade" brand, liquid multi-vitamins. You can find it on Amazon or at your local natural food store. It is a 'whole food' vitamin all natural. My daughter LOVES this 'vitamin' and it's liquid, and it tastes good, and it's easy to give it to them. Some kids just don't like vitamin tablets or 'gummies.'
Your son is clamping his mouth shut, and kicking his legs and shaking his head... just to eat? It should not be this way. Use a different approach... don't pressure. It is NOT about "control" or making a child know who's 'boss.' This is food...so, it should be pleasant. Not a big ordeal.
How about encouraging your child to help you cook? To have an opinion about what HE likes? Pleasantly. Teach him to ask for things... nicely. Teach him that if he likes eggs... he does not have to eat it scrambled...maybe he likes deviled eggs or easy over? That is fine. My girl likes eggs... but not scrambled. She likes it easy-over... and she's at the age now where she can even cook it herself. With my supervision.
If you offer him foods... and he has his mouth-clamping and legs kicking reaction... then just stop. Then you all go about eating yourselves. Don't bring 'drama' into it... just be matter of fact. Then when you are all done with dinner, for example, just go about putting away, washing the dishes, put the food away etc. THEN, even if he is not eating....just continue with the routine. THEN, if he says "I didn't eat yet..." or, "how come your'e putting away..." or, "where's my food?" Just let him know that "I'm sorry...I didn't know what to feed you... and you didn't want to..." or, "Well, then can you HELP Mommy problem solve this? What should I do? You didn't want to eat?" Then... see what he says. You are then TEACHING him that there is a process to things... and are teaching him HOW TO PROBLEM SOLVE. BUT, respond to him with a pleasant voice... not one of irritation or sarcasm, or "I told you so..." kind of intonations.
He is 3.5 years old... so bringing in this kind of thought process and problem solving ability is a GOOD thing to introduce to him. And later, it will serve him well. Instead of just clamping his mouth shut/legs kicking... it MAY teach him to express himself and to say what is going on with himself...to "HELP" as a 'team' and then to trouble-shoot the situation... thereby, making him choose what to eat, and in a different type of atmosphere, that HE is a part of. Not just a time of 'rejecting' food and having food plopped in front of him.
Also, for my girl, although she eats well... she will not eat if there is TOO much or too many things on her plate. It overwhelms her. Especially if she is tired. We know what makes her tic. So, I put her food on a smaller plate, with just enough on it, and then, she is able to eat without feeling pressured that she has to eat "everything" on it.
MANY kids just eat carbs... and dislike veggies or meat. It's a phase. If my kids are going through that phase.. .I make smoothies, and then they "eat" and they don't know that there are TONS of 'healthy' things that I put into it. :)
Well, just some ideas and what I go through and do.
If you are really worried about him though, ask your Pediatrician.
Good luck,
Susan