I agree with continuing to introduce meat...I don't agree with introducing chicken nuggets, hot dogs, or corn dogs at this age. You are forming their palates at this age--and all of those things in most forms are extemely high in salt. Why is that better than seasoned (not salty) small, cooked pieces of chicken, beef, or fish?
You didn't mention how many teeth he has? My daughter just got her front teeth at this age...she would chew all the flavor out and then spit it out because she couldn't get it small enough to swallow. It may just be a matter of dicing it extremely small and mixing it with something else.
I read that it takes more than 10 tries to get small children to accept new flavors. I find this to be true with my daughter. She readily eats many things now that took several introductions.
Animal sources are an important way to ensure that we get adequate amounts of vitamin B-12. Fortified foods can make sure you get plenty, you just have to be a bit more diligent to ensure he gets enough.
Some of the best advice I got about toddlers and eating was to not worry from day to day what they are getting. Look at what they are eating over a week or a month...I kept a journal for a few weeks just to keep track. I'm not normally that weird about things, but since she spent an enormous amount of time in daycare and with sitters, it reassured me that she was getting what she needed.