My DS also wasn't eating much solids at 13 months (I think contributing to anemia) but has since picked up. Things we changed: not nursing right before a meal, eating with us at dinnertime, self-feeding with a spoon (esp. yogurt, oatmeal, and sweet potatoes). Sometimes when he slows down on his own at dinnertime we cheat a little by giving him something to play with in his hands (like the food grinder) and then he'll usually be content to be spoon fed a bit more. I feel a bit guilty about this as I'd rather he self feeds, but this is how I get more high quality iron rich food into him (sometimes ground up and mixed with jarred sweet potato, which he loves).
Have you tried chick peas and lentils? And maybe hummus? I didn't see them on your list, but they're great sources of protein and iron. And how about olive oil? I douse most of he veggies in olive oil so he's getting some more good fat. Oh, how about sardines? Jeremy loves them, and they've got tons of good stuff: protein, calcium, omega-3 fats. There a bit on the salty side, but I think its okay occasionally if the rest of the diet is low salt.
Good luck, I know its tough to see him losing weight. But Jeremy turned a corner on solids just a few months older than Luke is now.