this problem is faced by many parents worldwide and its really troublesome feeding your child.
but its really nice seeing a 700 grammer grow. i appreciate your effort and patience.
the problem of chewing and spitting out food is very common with children, and the main reason is they don't have liking developed to most oof the food items.
what you should try is to give him somewhat bland food items that is without much sugsr or salt in it and let him develop the taste for the actual food he is eating, gradually you can add sugar and salt to his taste and check for his liking for these things.
you should give him only one kind of cereal or pulses to start with and not a mixture and can make a paste of it to eat and gradually increase the consistency rather than switching directly to solid food.
many a times adding some butter or oil can make food more palatable, so you can try this.
if nothing works then you should get him checked by a pediatrician
ir surgeon for any kind of obstruction in the esophagus
that may be hindering the passage. as there are many conditions that may cause this like strictures or bands that may be present since birth or may be acquired.
regarding falling ill on and off, if its some common cold
like illnesses than there is nothing to worry about but if its serious illnesses like pneumonia
than it may need evaluation, furthermore its more common in children born preterm for some reasons. so you can get an evaluation done.