You said he likes apples--and *sometimes* bananas. So buy apples and bananas and keep them available. Fruit bowls are good for this.

I have kids who love fruit and like some veggies. I buy a variety of stuff and change it up. We tend to pretty much always have bananas and apples and then *either* oranges, pears, peaches whatever's on sale, and a few times a month (I TRY for once a week but doesn't always happen) a "treat fruit"--a melon, pineapple, pomegranate, mangoes. (read: when Mom goes to the store and sees a sale on one of these)
I'm actually working on offering more veggies, part of it is just remembering to do it. I go with offering right before dinner (who cares, they're eating something healthy, and they are starving, so they are more likely to go for it) and sometimes alongside another snack. (like crackers and carrot sticks and dip)
I notice since I put forth the effort to try and offer more veggies (have them present at *almost* every meal) my kids are trying and eating them. DS1 hardly ever ate salad, now this week I have seen him eat almost nothing *but* salad at 2 meals.
My kids serve themselves as much as possible. I really think this helps.
Also, personal opinion here, but I don't think that canned fruit, as long as it is canned in 100% juice and not corn syrup, is "bad". It is still fruit. And it might even be a "gateway" like "OK I liked those canned peaches, maybe it won't be so bad to try this fresh peach..."
*I* was *Extremely* 'picky' as a child, things like that *eventually* worked for me, that kind of logic. (OK this is XYZ food and I had it this way and liked it....maybe I can try it here too) The thing was it was mostly in my own head, not anything anyone tried to get me to do.
Oh and my kids like to pick out stuff at the store. Most kids do I think.
So, in a nutshell...i wouldn't worry about it. After all, many fruits and veg give the same vitamins, I am sure he is getting what he needs. But in the interest of eating a variety of different foods and learning to like things, I would
--Keep buying the apples and bananas he likes.
--Let him pick out one thing to try at the store. (if he's willing. Make it a family thing and not just about the fruit and it'll probably go over better.

)
--Simply present it and then back off. Maybe since he's only 6 a little talk about what it is and if there's something similar he has had and liked might make trying it less 'scary.'
--Oh and I forgot this from when I taught preschool--Spitting out is OK! I've seen many kids who never tried anything before start trying new things when they were given the control over whether or not they actually had to swallow it. Many kids end up liking it right away, some do the 'spit it out' thing even several times the same item's presented.
eventually they DO expand their palates, I am living proof,

(as a person who used to *only* eat canned chili, mashed potatoes, french fries, and Cream of Wheat...who now eats pretty much EVERYTHING from ANYWHERE and the hotter the better!)