As mentioned by you,at 3 years,his weight is 17 kg,which is VERY good,so in my opinion,his oral intake should be the least cause of worry,relax!! Children are fussy eaters,some more than others,do NOT force feed him,that's the case with you,I think. If he's active and playing normally and gaining weight over a period of time,you're doing FINE!!

He may have some acidity issues. Kindly discuss this with your pediatrician so that they can come up with a customised diet plan for your child