Could be that your son is asserting his growing independence in the culinary department :-). I was also mystified when my girls were around that age and suddenly things they wolfed down weeks ago were going untouched .... later I read in Ellyn Satter's "Child of Mine, feeding with love and good sense" that it's VERY common for children at this age to be more assertive in the food department. Her advice is to continue offering a variety of healthy items even if the same item was just rejected hours ago, because oftentimes it'll take 10 attempts before your child will allow the item to be left on his plate and maybe another 10 attempts until he decides he wants to put it in his mouth.
Some things my kids liked:
-slice bananas into discs about 1/4" thick, dip them in pancake batter and cook like regular pancakes (make a bunch and freeze the extras, they reheat quickly in the toaster oven. sprinkle a little wheat germ on the toaster oven tray to prevent the pancakes from sticking). Tasty and the perfect size for toddler finger-food. My girls are 6.5YO now and they still love these - the banana takes on a slightly pudding-ish texture from the cooking. Apple slices also work.
-save the crumbs from the bottom of your bag of cheerios and roll finger-food size cubes of fruit, cooked veggies (sweet potato, carrots, etc.) or tofu. The "cheerio dust" makes the cubes easier for toddlers to pick up.
-for snacking, the air-puffed grain cereals are good to have on hand. My girls faves are the puffed corn and the puffed rice, the puffed kamut is also nice and these are all good sizes for little fingers to pick up (and these have no added oils, sugars, or preservatives. another bonus is that they 'squish' instead of crumble so when you accidentally step on the ones that fell to the floor, there aren't a ton of crumbs to sweep up). We usually get the arrowhead mills brand at Berkeley Bowl for a little over $1/bag.
-hummus - they'd actually eat it straight out of a bowl or on crackers, rice cakes, or veggie strips
-cooked beans. I get dried beans in bulk at Berkeley Bowl, soak overnight and boil, then freeze in small portions for cooking or for snacking. My kids liked snacking on cooked garbanzo beans best.
- make "sweet potato fries" by slicing sweet potatos into french-fry length strips, coating lightly with cooking oil (dust with a little cinnamon and nutmeg or salt, depending on your preference), and baking (depending on your oven, about 15-30 min at around 400 degrees)
-if you have small cookie cutters, sometimes cutting cheese and bread into fun shapes will make them more appealing to little ones. Experiment with other shapes and textures too - kids who don't care for carrot sticks or cheese squares sometimes find that a mound of shredded carrots or thin strips of "cheese like noodles (my girls' name for cheese run through the shredder attachment of my Cuisinart)" looks irresistible.