Bingalau, please explain a bit more what your son wants to achieve? Being unable to read minds is a bit of a nuisance. So please expand on what you are after as its difficult to craft a simple answer, if there is one of course.
Does he want to migrate all his emails across to OE (Outlook Express), with his email addresses, or what?
I note that replies suggesting the use of a conversion program was too complicated for you. click here
I am not at all knowledgeable about Incredimail but if you provide more details of what you seek, then there will probably be more chance someone else might understand and respond.