9mth old always constipated w/ every solid food he eats. Can something be wrong?

My son is 9mths old and ever since he started solids at 7 and 1/2mths old he is always having trouble going to the bathroom. Always straining, and usually nothing is in his diaper. He really only eats fruits, which is mostly pears and peaches to help with constipation. Hardly any bananas or applesauce. He is drinking a little apple juice with every meal. Which he only eats once a day and probably half a jar at most. He does snack on the graduate Puffs (sweet potato). But that consist of maybe a handful through out the day. I don't know what to do.. He is exclusively fed. He won't take prune juice or eat prunes... Can something really be wrong? I am unable to eat dairy b/c of sensitve stomach. He was on reflux meds for 4mths, but took him off and is doing great with that... When he does go to the bathroom its hard pebbles. I called the doc. a month ago and was told to give miralax. I've only done that twice. I hate giving him meds. If you could give me any suggestions, it would be helpful. We have his 9mth appt. tomorrow.