This is neither a Green Cheek nor a Maroon Bellied Conure, but a Black Capped Conure. I dont think Black Capped typically have the red on their bellies, so it is possible he is a hybrid of a black capped with a maroon bellied or green cheeked conure. What color is his tail? The main difference between maroon bellied and green cheeked is the tail color. A mbc will have a green tail, while a gcc will have a red tail (except with turquoise). Also, a maroon bellied will have a lot of green on the head and neck where a GCC will have mostly grey. So this is either a black capped, or a hybrid of a black capped. Where did you buy him? If you bought him from a pet store or a "hobby breeder" there is a chance that they do not realize they are creating hybrids/selling birds under the wrong name, and they really should be informed.
Never mind, it sounds like you got him from a private party? I am glad he has come to you. It sounds like you have him caged with your cinnamon now, is cinnamon the name of the bird, or do you mean a cinnamon GCC? Please be careful not to allow him to breed with your GCC if this is the case. Because the Pyrrhura conures often look so similar (if not we would not be having this conversation at all!) it is very hard to note the small differences in a hybrid sometimes. Since 1992 it has been illegal to import most species of parrot into the USA, so as far as the gene pool goes "what we have is all we get" and it is important not to pollute the stock we have by accidental hybridization.
Best of luck with your little guy! I hope to see more pics and updates!!