lots of good advice (very interesting about the probiotics)...my DD is 21 months and very picky, too.
A couple things I've noticed--she tends to eat differently depending where we are. If I pack a meal or a snack for an outing, sometimes she'll eat the thing she's refused 20 times at home. Not sure why, but...
Also, can you get DS involved in preparing meals? DD loves whole eggs (she has wood toy eggs), so I let her help crack the real ones, mix and watch me scramble them. She ate 5 bites (huge for her!).
Maybe try the nibble tray idea....put a variety of finger foods in an ice-cube or muffin tray (or just put them on a plate) and let DS snack on them throughout the day. This works well for my DD, especially since she'd rather stand while eating (I put the plate on a low ottomon in the living room or in her Learning Tower).
We're eating lots of homemade popsicles right now. It's not the ideal food, but I can sneak in yogurt, coconut milk, flax, and other things she won't eat otherwise. I use the smallest amount of maple syrup I can get away with to sweeten them.
It's really hard not to worry. I swear, DD only eats corn, peas, pine nuts, cheese, and sunflower seeds on a regular basis.
: It's tough, but I just keep offering and try to stay low-key about it all.