It appears to be sickle/beta+ thalassemia, which is not as severe as the beta° sickle combination. There is no speciific diet, but it is wise to follow a diet high in antioxidants, so fruits, veggies and beans (no broad or wide beans) are an essential part of that diet. I would suggest supplements of folic acid (most important), natural vitamin E (no synthetic) and B vitamins. For now, a multivitamin might contain sufficient B vitamins.
This combination of sickle cell and beta thalassemia is quite common, but the severe form is the beta° form, where you would see little to no HbA. Your child will never have the ongoing severe condition, but be aware that moving to high altitudes can set off a sickling crisis, so keep that in mind for the future.