I don't know of any 'specific' gramatical reason why you can't abbreviate a word at the end of a sentence, but it's frowned upon. It reads awkwardly/unnaturally. If I had to guess it's because it reads more like something belonging to Henry. It is Henry's, for example. If you have this type of sentence you can either: - Use the non abbreviated version: Tom is tall and Henry is too. - Restructure your sentence a bit: Tom is tall and so is Henry. Or, Both Tom and Henry are tall.

The best answer is "Tom is tall, so is Henry"

You need to say 'Henry is' here. You could say 'and Henry's tall too', but not 'and Henry's too'. The contraction shouldn't come before a word like 'too'. I'm not sure how to explain it in more detail, but hopefully that's enough :)

No