My Son Is 13 And Went Through Puberty Quite Early

my son is 13 and went through puberty quite early - he grew very fast in the last 12 months also. His voice has been continuously croaky for 18 months and he has to strain really hard to be heard. Ent says he has
swollen voice box but nothing else wrong. What can he do to remedy this? He can't not speak! He is getting very embarrassed by it as no one else has this problem either. thank you