I would say: "He thinks life is interesting, and so do I." It means: "He thinks life is interesting, and I think life is interesting, too." The other sentence with am is not grammatical in standard US English. To get your second meaning, I would use the non-elliptical sentence you have in your explanation. It still sounds a bit funny yo me, and I would probably rewrite it to something else.

Ceci n'est pas un seing.