Are there any dietary supplements to support hair growth?

None are effective: There are no vitamins, herbs, supplements, nor foods that are proven to slow hair loss or improve hair growth. Maintaining a balanced diet of healthy foods, and taking one multivitamin + minerals tablet each day, should keep a person in normal, good nutritional status... So his scalp and hair should be as healthy as they can be. A doctor can evaluate unusual hair loss for hormonal problems, etc...

Only if deficient: Deficiencies of iron, protein, and calories can lead to hair loss, or lack of growth. Biotin deficiency is pretty rare (ubiquitous in diet, made by gut bacteria). Well balanced diet should be enough. May need supplement of above if found. Lots of causes for loss of hair-hormonal, stress, illness, pregnancy, alcohol, smoking, drugs, etc. No supplements proven to work.