It is a toddler phase. It will end. But like you I'd be worried about setting up a pattern I wasn't keen to continue.

Totally agree with Kelebek about consistency. I'm up to my second quirky toddler and it's the one thing I have learned. I wouldn't offer a sandwich nor the option of pureed veges TBH. He's not trying anything because he knows you will give him what he prefers. Same with cooking him a different dinner. He won't starve, I promise. We only serve a different dinner if it's something they seriously cannot eat like a vindaloo.
I totally get that you're happy with the pureed veges because it means he's getting something 'good', but TBH most 12 monthers don't need mashed food, so a much older child (without any side issues) certainly does not.
I truly think you're going about the enthusiasm and helping thing exactly the right way. Toddlers need up to 10 exposures of new stuff to be familiar with it, then there's the same amount to decide whether they like it or not. Just because your little guy doesn't want to try it right now in his "I'm going to dig my heels in cos I'm a mighty toddler!" phase, doesn't mean he won't in the future.