I wouldn't use "under-confident."
I'd either do as Su.Ki says--use a plain, clear phrase: "he lacks confidence," "he isn't confident," or look for a completely different word that has the opposite meaning.
The ones that come to mind: "timid," "uncertain," "unsure," "insecure," "vacillating."
Let's see what an online thesaurus suggests:
http://www.thesaurus.com/browse/confident
Of the ones it lists, I like "doubtful" and "unsure."
November 25, 2014
0
2
1
You can, but it's not very common.
You can also say 'He lacks confidence'.
November 25, 2014
3
1
Still haven’t found your answers?
Write down your questions and let the native speakers help you!