He used to eat everything

DS used to eat everything, but has become a very picky toddler. I can no longer get him to eat any berries (even blueberry muffins or pancakes), lots of fruits he used to eat (apricots, nectarines, cantalope), anything that resembles baby food (incl. yogurt, mashed potatoes, sauces), etc. Except for watermelon, he only eats the year round fruits, and I feel like he's missing out on so many good flavors and nutrient dense foods. He always wants cereal, crackers, and granola bars (Easy to eat and hold foods are faves, plus these are the snacks we always had for the car and park, when we'd be out of the house for long periods of time.) He likes pasta, and we usually make a veggie-heavy sauce with tiny bits of veggies (indistinguishable in the sauce) but he won't even eat that now. He'll literally throw it if it isn't just pasta with parmesan cheese. He loves mushrooms, but if I put them in the plain pasta, he won't eat it.

I know it's normal for toddlers to become picky, but I'm trying to figure out if there's anything I can do to encourage a greater array of foods. I don't want to make a big deal out of it, but my gentle encouragement isn't working. I've told him that he just needs to try it, and if he doesn't like it, he won't have to eat it, but he won't even taste it. I've tried putting various new things on his plate, and he won't touch them. I've told him he can't have crackers (or whatever else he's asking for) unless he tries an apricot, for instance, and he lets it go. I've tried peeling and cutting things up just like his faves, and nothing. I try to pair things with peanut butter, which he loves, but if it isn't an apple, he'll ignore it and just eat the PB with his fingers. I really don't know what to do. Any ideas?