An 18 month old probably doesn't want to sit through your entire meal. But certainly he can learn to sit in his high chair for the start of the meal. Here is what I would do:
1)Get a kitchen timer. Show your son and set it for 5 minutes. Tell him that when it is dinner time, he will be in his chair until the timer goes off, then he can go play, but mommy and daddy are still eating.Gradually increase the time you set the timer for and your son's time at the table with you as he gets used to it or doesn't seem done.
At first he will probably fight this- but I would not give in. He is just starting the age where he will CONSTANTLY test you on things. Just because he is done and wants mommy to play doesn't mean you should not get to finish dinner! Just repeat the drill gently over and over and over. He'll get it eventually- he just wants to try and be the boss :)
2)I agree with your doctor- he will eat when he is hungry. But don't let him graze on juice and crackers, etc. all afternoon and then wonder why he won't eat dinner. At dinner give him VERY SMALL portions of what you are eating. I mean, just a bite or two. Encourage him to try each thing or eat the things you know he likes, but other than that, don't sweat it.
It is your job to give him enough food to taste each thing- and ask for more of what he likes, not a lot of each thing. A lot of moms give their kids too much and then worry they don't eat. Just a bite or two at a time is plenty to start with.
3)When the timer goes off, don't push him if he hasn't cleaned his plate. He is still little and you want this to always be a positive thing- sitting down with mommy and daddy at the table is fun!
Take him down and set him up to play with something- you could even have a special toy he really likes that he only gets to play with after dinner.
If he whines or grabs your hand, just keep gently disengaging him and telling him that mommy needs to finish her dinner too. You can also give him the option of getting BACK in the chair and joining you until the timer goes off again! By this time, he may be bored with playing and that might appeal to him.
Also, arrange some special 'daddy' play time after daddy finishes dinner. Just be sure to always tell your son "Daddy will come and play with you as soon as he is done eating, but only if you can play with your blocks for a few minutes by yourself". Remember to keep the time frames SHORT- a few minutes is a long time to your little guy!
The important thing here is to get him used to the idea that the family sits at the table, everyone has a plate, everyone eats some of the food.
You are training him early in a good habit that will not only last a lifetime- but also, trust me, make your life easier at home, in restaurants and at other people's houses for years to come!
This is what we did with my son- I got this advice from a mom I knew who had the best-behaved little kids I had ever met. It really worked for her and for us as well.
When my son was a toddler, he would sit in his high chair on his own for an hour looking at board books. Everyone couldn't believe how 'self-soothing' he was. He was always so good in restaurants - we could take him anywhere and always got so many nice comments about how good he was in public! (not to say he didn't have his moments or throw tantrums, but all little kids do!)
As my friend told me about her kids being good at the table "We just never let them learn it was ok to be bad and throw sugar packets or food around!"
Good luck!!