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."
25 novembre 2014
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You can, but it's not very common.
You can also say 'He lacks confidence'.
25 novembre 2014
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