Brief Answer:
Improvement takes time.

Detailed Answer:
I understand that you must be worried because as a relative you expect/hope a visible effect after treatment administration.

Guillain Barre during the first weeks has usually a progressive course. The severity and duration of symptoms vary from one patient to the other some remain ambulant, while others progress up to the point of needing intubation to assist breathing.
So it is difficult to assess in an individual patient how things would have gone without appropriate treatment, immune globulin in your case. While you feel disappointed because he is not improving the fact that he has deteriorated might be owed precisely to the immune globulin.
That is why efficacy of such treatment is evident only in studies with large numbers of patients where it has been evident that those patients who got immune globulin (or plasmapheresis which has similar efficacy) have lower severity and shorter course.

In the meanwhile you have to understand that some damage to the nerve has already happened, immunoglobulin has hopefully slowed or stopped that advancing process, but regeneration of the nerves is a process done by the patient's body itself and can take months.

The majority of patients recover very well. Since he is young and has taken appropriate treatment you can expect him to have a good outcome, but patience is needed.

I remain at your disposal for further questions.

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