Poor babe! Here's a shot in the dark just to add to your bag of info.......sensory processing problems can sometimes cause problems with children eating. Mostly you'll hear about it when moms are talking about how picky their toddlers are, how they'll only eat one food, one texture, food at a certain temperature, only sweet, salty, chewy or crunchy, or moms will notice that certain foods cause changes in behavior. If that rings any bells, you can do some online searches or ask about sensory integration disorder or (the name keeps changing) "Sensory Integrative Dysfunction". You'll most often see symptoms in conjunction with other, more common symptoms of the disorder, and, should it be at play, there are many things to do that can help.
Also for your consideration: allergies. Maybe you've already made a note of any reactions to food/drink, though.
Common sense and other moms here would suggest teething, reflux, and your basic, "I'm a toddler now and I'm too excited about the world to sit still and eat!" What's going on around meal time? Have there been any big changes in his routine or his life in general? One of my kids was low on the growth charts until she turned 9, when she had a huge growth spurt that has lasted a few years now. In my experience, Ensure, although sugar-laden, can be a literal life-saver until you can find the root of the problem, and the less stressful meal times are, the better. Just all, pardon the pun, food for thought. I know it's no laughing matter - hoping to ease your mind a little. I also know that you'll keep searching for answers til you find one, and that that makes him a lucky boy. Hugs if you want them to you and your little guy, and may his appetite return soon!