I wouldn't worry too much about fruit. Its far preferable to tantruming for biscuits or sweets. If you google it, I think you'll find the sugar in fruit is much healthier than the sugar you find in biscuits, cakes etc.
In terms of avoiding an upset stomach I limit portion sizes of things like grapes, strawberries & citrus fruits.
To protect teeth then always offer water or milk or a cube of cheese after eating fruit to rinse the teeth & neutralise acid. Never brush within hr of eating as enamel is temporarily weakened.
These are just precautions however because I honestly think if you brush their teeth twice a day, they drink water rather than juice then their milk teeth will last just fine.
I do, however, understand your concerns, as I went through similar with ds1. He had healthy diet, teeth brushed etc, drank mainly water, but he used to have bottles of milk during the night & I had read it could cause tooth decay..... I panicked, took him to dentist who pretty much said to chill out, his teeth were perfect, they are not permanent teeth and gave above advice. (I did go cold turkey on bottles though).
Ds1 is now almost 5 and still has snowy White teeth.
I wouldn't worry about fruit making anyone fat either, I dont think people get overweight by eating too much fruit. It's low calorie.
Does he also like things like cucumber, tomatoes, celery, sweetcorn, carrot?
Generally though I'd just be happy he is opting for fruit with it's high nutritional value as opposed to biscuits or sweets.
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