I love
Sam. It's great. And very sweet.
I don't like
Samuel as much, but it's the best
Sam- name and I do think
Sam is a bit nicknamey so I would probably go with
Samuel "
Sam".
The only reason I would put a full name on the birth certificate is because he might want a full name when he grows up.
Samuel sounds more professional than
Sam does. And there's always a possibility that he doesn't want to go by a nickname, that way
Samuel is a great name to fall back on.