Without having a blood test you can't know for certain if he's calcium deficient. I'd be cautious about giving him anything supplemental without knowing for sure. He will get what he needs if you transition him over to a wide variety of veggies. Fruit is high in sugar, so it's best not to give him primarily a fruit diet.
If he's not eating veggies, and if you have access to blended vegetable juices, you could try giving him his veggies via a drink. My flock love to sip veggie juice off a spoon!
Spinach binds with calcium, so it's best to avoid spinach.