Grethe,
I think that you have had many good suggestions here and I have but a few thoughts to add.

The forehead looks a bit distorted so I straightened the reference to make the top and button sides parallel and also widened the picture.

I think that you could lighten the shadow side of his head and also darken the collar/shirt and also the shadow side of his white coat. I have done a little here and there, you'll see where.

You have done a very fine job for a first portrait, take your time with it.

Debra, thank you very much for the information and your helpful suggestion, very much appreciated.
Allan, thank you so much, it is beginning to look much better.already.
I will go for most of the ideas here.
Yes, I will take my time to get it right so I can feel comfortable about this work. He is also a good friend and allows me to set the deadline as I wish. although he has seen it and wanted to put it on his wall, right away. But I am glad I had second thoughts about it, and so happy to be part of this forum.