Eight is still too young to be worried about that, at least as far as your son is concerned. With regards to hair growth, most boys will begin to enter puberty no sooner than the age of 10. Any changes in hair growth and texture would not be expected until at least that age. In the specific case of your son, you should still have a few years left until you need to worry
about his hair growth being too slow.
The two major contributing factors to your son's growth at you and your husband (good nutrition is also important, but most children in the United States are well fed--we usually have problems with being over-fed!). Genetics controls to a large degree not only how tall and how heavy your son will be, but also the likely age at which he will enter puberty. A great place to find some guidance on your son's growth would be a growth chart. These can follow the height and weight of your son over time, and will give you the chance to make adjustments to diet as needed. They will also provide some limited information about projections for how large your son may be in the future. Finally, this question is the perfect question to raise with your pediatrician
in an annual physical
exam or checkup. Your physician can give you tailored suggestions and answer your questions much better.