You might try a different kind of sandwich tomorrow and see what part of it he was so interested in. If he likes a different kind of sandwich, then it's the texture, but if he's not interested, then it's probably the salmon. Other tasty sandwich alternatives...peanut butter, jam, hummus, banana or avocado ripe enough to spread, veggie or fruit puree, cream cheese, or grilled cheese. If it's salty he's after, you can mix just about anything with a bit of salmon. It may be disgusting to you and me, but toddlers often eat the most horrible combinations of food (mine does, anyway).
Oh, and marmite's pretty salty too, isn't it? That might make a good sandwich!
