None of mine are big fruit eaters the only fruit they have ever touched is strawberry, they mostly eat veggies as long as he is eating other healthy foods I wouldn't be to worried about the fruit alot of tiels are not big fruit eater's. Have you tried other fruits besides apples and grapes maybe present them in different ways to see if he will try them maybe try not cutting them up into tiny pieces give bigger pieces mine will only eat the strawberries if they are bigger pieces that they can pick at.