I'm so glad you've decided to take him. There aren't many people who would take on a dental bunny, so I'd like to say a big 'thank you'
If it's his incisors, I very much doubt that a change in diet will prevent his teeth overgrowing...chances are his teeth just don't meet properly to wear down. Keep a close eye on his teeth because they may need doing every couple of weeks at worst. Also keep an eye out for signs of discomfort/pain,discoloured teeth or pus because one of the causes for malloclusion is tooth root infection.