My Son Is 6 Years Old. We Adopted Him When

Question: My son is 6 years old. We adopted him when he was 20month old from South XXXXXXX He had a malnutrition history before 1 years old (after that, he had very good care). His upper teeth can not close properly to the lower teeth since we had him. This certainly makes biting difficult, such as biting an apple. This also affects the clarity of his speech (he is language wise articulate and talkative :-). The inside space of his mouth is very big/high (sorry I don't know the best term to describe :-). His teeth in general doesn't look very well grown. He sees dentist regularly and no other issues found. The dentist thinks his problem is mainly du to malnutrition history. Now that he is 6, he is in the middle of tooth changing. I attached some photos here.
Our regular dentist says surgery can be done for him once he grows up but didn't tell us how old is the proper timing. I just hope to check with you on what kind of surgery can be done so his teeth can close properly. and what age is appropriate. He is growing up and changing from a silly boy to a boy with gradual self conscious in his appearance. So I just want to make sure we don't miss the best timing to fix his problem.