You do what you need to, to feed your child. But make it peaceful.
YES, it takes time to feed a baby/child. This is a given. They will need help to eat until they are much older. Eating/meal times take time with kids. Yes, it can be frustrating. YES, MANY kids hate high-chairs. Perhaps, just let him stand on the regular chair... many kids like doing this instead. Or, get a "toddler chair" that you can put on top of your regular chair, like a booster seat.
Do NOT compare your child to other kids or Parents.
ALL kids have eating phases THROUGHOUT childhood... at every age stage, they go through this. It's normal.
Pick your battles. Don't give him food hang-ups. Encourage but don't "punish" with food.
No child is a "saint" when it comes to eating. Don't feel stress about it. They ALL get picky at some time or another. 2 years old, 3 years old, 4 and 5 and 6 and onward, you will encounter food and eating "issues." So don't burn out on it now.
If Doctor says he's fine, then fine. Unless he has true medical problems or concerns, I would not worry.
As a boy... don't give him a complex that he is "small." Many times, people expect "boys" to be bigger and huskier. Never
mind that. He will grow and fill-out as his body is ready... just keep trying to offer him variety. My Mom (Grandma) used to always comment that my son is "skinny/bony for a "Boy." I told her to just stop it... he is the way he is and he's healthy and happy. It's fine. I will not have her telling my son that he is "skinny" and live with expectations that a "boy" has to be "big" husky. Many times, boys will grow later and fill out. It's fine. Size is also genetic. My son is in the 95th percentile for height... and in the 25-50th percentile for weight, and 75th for head. The Doctor says he is perfect... better than having an overweight child, and this is his "natural" build.
A child's taste-buds are also still developing... so they "acquire" tastes or preferences. Keep that in mind.
My son is picky and "skinny", but he is healthy and tall and developing fine, and I give him Doctor prescribed vitamins (Polyvitamin drops with iron) and Pediasure. It's okay. But, he will ONLY eat if he is truly hungry and he will ask too. That is the way he is, versus my daughter will eat with no problem and she eats anything I give her.
For my son who is VERY picky, I just make sure he has food available to him throughout the day... I make a variety of bite sized and 'safe" foods for him, put it in bowls on a tray and leave it on our living room table since this is where he can reach... and as he's playing or at "meal time"... he will either help himself or sit at his toddler sized chair and eat... at will. I 'help' him and supervise him... but I don't force him. He does not do well with forcing him to eat. He also has his own rhythm in eating... he is a "grazer." He is NOT the type that eats a ton all at one sitting (unlike my daughter). So he is just different. I know his pattern. I go with it, and before I know it, he has consumed a good amount throughout the day. He grazes.
I also make healthy smoothies for my son, and soups... since he likes that. I give him a straw and he drinks (ie: eats) a lot this way.
My son is also very active, he will NOT sit continuously at the dining table. So, I don't battle about it. Many times, we also like to sit outside and eat on the patio... I've found my son eats more when we eat outside.
Yes, as your Doctor said... some kids just don't eat a lot. Just like adults, right? Some adults eat like birds and are VERY picky... same for kids.
Serving size at this age IS in terms of tablespoons. So at least your son is eating lunch and dinner. Maybe later, he will work up to more. But for now... this seems to be his pace.
Just do not compare him to your friends kids.