I congratulate you for having a qualified son.
This problem is not stammering- stammering starts in childhood and mostly resolves by that age, if at all it persists then it is present all the times.
Your son is finding it difficult to cope up with the stress
of public gathering- this is a universal problem for the beginners and the more they deal in these seminars the more they get confident. This problem is called Performance Anxiety
, more anxiety more stress, more distractions.
Remedy is to read the presentation slides before actual event at least twice after the presentation is finally ready. If possible present it among his close friends as a rehearsal.
Alternatively he may present before a good sized mirror also.
He should be ready with the welcome note in advance and must practice it by actually loudly speaking multiple times as if he were in actual seminar addressing people, because a good start is half done!
If still the trouble remains then there are very good medicines which if taken in Regimens will support a less confident person.
That should help him.
All the best for the next seminar!