Can Forms Central be used to create (fillable) school worksheets for my son, that he can then open and type responces to? That is the only thing I need it for. For instance, I would like to make worksheets for spelling that would ask him to type certain spellings words (from a list) where long a is spelled ai, et cetera.
Sure - that sound like a very achievable form to create using FormsCentral. You can/should give it a try by creating a free account and either creating a new form from "blank" (new custom form) or starting with a related Template you would modify. You can see if it would work for you, you can have one form at a time in your free account, you can always delete the one you try out and then try another.