It shouldn't be a problem as long as upper and lower teeth don't rub on each other, which could lead to early tooth loss. When he'll grow up some more it should get better. Must be careful and look in his mouth when he's teeth will start changing, for teeth growing the wrong way, retained canines-see photo(permanent canine erupts while the "baby" canine is still attached), and other teeth problems.
If an under bite is noted before the permanent teeth erupt, treatment may be helpful. Removal of primary teeth from the shorter jaw that interfere with forward growth, may allow the upper jaw to lengthen unimpeded. This procedure called interceptive orthodontics will correct about 50% of minor jaw length malocclusions, by the time permanent teeth erupt. Extraction does not stimulate jaw growth, it only removes a mechanical barrier of the growth process.
Teeth that are crowded or tilted at abnormal angles can result in early onset and increased severity of oral infection, may damage the soft tissues of the mouth, due to sharp teeth that penetrate the unprotected gum and mouth tissues. In addition, the lower canines can erode through the hard palate/roof of the mouth, causing food to enter the nasal cavity, excessive wear when abnormally aligned teeth grind against each other. Such abrasion frequently wears through enamel, causing a weakened tooth to fracture and expose the root canal system causing pain, and early teeth loss.
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