Praise be to Allah.
If there is the hope that it will grow by itself, he should not try to treat it, because this is not a defect. Often when young men start to grow their beards, the beard does not grow evenly, so he should wait.
But if it is a defect, in the sense that we know and are sure that it will never grow by itself, then there is no problem with treating that, so that the rest of it will grow, especially if it is unsightly.
But if it is not unsightly, then the best is not to treat it with anything, because I fear that that may come under the heading of adding hair extensions, for which the Messenger (blessings and peace of Allah be upon him) cursed the woman who adds hair extensions and the one who asks for that to be done. End quote.
Shaykh Muhammad ibn ‘Uthaymin (may Allah have mercy on him).
Liqa’at al-Bab al-Maftuh (2/400).