My baby lost most of his hair at 6 mo. Why? When will it grow back?

Transitional hair. Newborn hair will usually fall out gradually over the first year & just as gradually be replaced with a thicker hair. It can be quite noticeable, producing a bald appearance, yet a close examination of the scalp shows emerging hair.Eyebrows & lashes are not affected during the process. Some cultures appear less affected than others (darker hair<< lighter groups).
Alopecia universalis. Complete alopeecia is usually autoimmune. It can be dealt with cosmetically in the short term, but i would see a dermatologist or immunologist. It often goes away, but in many cases not.