Just like people they have patterns that are genetically predetermined. If he is not rubbing it out then no. No fixing it or making it grow. You can encourage what he has to grow and meet his full potential if he is malnourished in some way but if he is on a good feed and getting his nutrients met then there will not be a significant change. Considering the rest of his mane and tail are full and thick as well as in good health then perhaps a comb over would work.
Some horse people change their horse, they change their tack and discipline, they change their instructor; they never change themselves.