Oh then I would definitely not take that to mean the investigation is somehow limited to Don Jr. They are just getting started.
I wouldn't expect Trump to be asked for statements for months because what they are going to do now is have the forensic accounting firm Vance hired go over his tax returns and the finances of his business and construct what they think he actually owed in taxes and compare it to what he claimed. After they are done with that, which will take months, they will then start getting statements from Trump Org. officials/Trump as to why the discrepancies exist, and go from there.
Again, the Manhattan DA is investing a shitload of time and money into this case, they clearly think they have something and it likely involves Trump himself.