Yup, probably a phase. My son has done that with almost every food group now. With him, it lasts anywhere from a day to about a month. At one point all he wanted was prunes! My best advice for this is to be patient, to see about a Rx multi-vitamin if his diet is not well-rounded enough in general (look for him to be getting a balance of each food group over a period of 3-4 days), and to serve the fruits with other foods that are compatible with them- healthy apple pie recipes, berries in a parfait with yogurt and granola, pineapple on a hawaiian pizza or on honey-glazed ham.
[I recently discovered that my son loves limes: you can make a thai coconut soup to serve with a slice of lime and tortilla chips. Bizarrely, my son ate lots of stuff if he was allowed to rub the slice of lime over the foods first. (If you want the soup recipe, email me, and I'll copy it out for you).]
Strawberries or blueberries on cold cereal with milk. Peanut butter poppers include raisins, powdered milk, pb, oats and honey. (Email for the recipe.) Bananas layered in jello, or in pudding with vanilla wafers. Applesauce coffee cake (email for recipe).
Just take whatever fruit he likes and make whatever you can think of to go in it, on it, or around it, or to mix it in. Make sure he sees you or better yet, helps you make it so he knows the fruit is in there. (hope this helps. it's what I've been doing for over a year now. The idea works with any food group).