Re: Expanding his diet...
So at 17 months your child mainly eats pureed foods? It sounds as if he has a feeding delay. My toddler also had a feeding delay and he had the pleasure of going to feeding therapy. In our case, he was tongue tied and for a very long time was physically unable to lift his tongue and swallow. After he had tongue tie surgery, it was a mental thing... his confidence was very low and he thought he couldn't swallow although he actually could.
For the most part we did a lot of modeling with him in therapy and at home. A LOT of verbal praise and try to keep meals low-stress. Can you tell if your LO is gagging on any foods? Does it seem like he has some desensitized parts in his mouth?
So your LO likes cheese? If he eats cheese sticks, then try slim jims. They are similar shapes but different textures. We worked a lot with kid-friendly foods at first and slowly worked over to 'things that we were eating' on his plate too. Mine also had a hard time with meats, but he liked chicken nuggets and deli meats.
Agh there is so many things. How does your LO do with 2 textures at the same time? Feeding is all about textures. You should do 'messy play' with food as well, use puddings and let him get his hands in it.
If you're really concerned bring it up to your pedi. We found feeding therapy very helpful, and we only needed to go about 6 sessions until I felt we could figure out the rest on our own.