My boy also like that initially. Only improved after 1 month plus. Maybe instead of pushing the spoon in, put the spoon in front of him and open your own mouth so that he can see you opening your mouth. That's what I did.
I think 3 heaped tablespoon is enough. My boy is taking 3 heaped tablespoons of cereal for dinner.
When he just started out, he took 1 teaspoon, followed by 1 heaped teaspoon, 1 tablespoon, 1 heaped tablespon and so on.
As long as he's not displaying signs of hunger after food and can drag till longer to the next milk feed, you should be giving him enough.