your horse must worked over the back and topline using the hind quarters. The head will raise naturally with practice. Lots of transitions and engagement of the hind quarters is needed. Rider must also be balanced and upright x
Ok I understand, if his head is low but he has balance and his hindlegs are engaged I don't see why it should be a problem.
However if it makes him lose his balance it's more problematic.
You should probably shorten your reins and make him more in front of your legs.
You can also work with bounces, it really helps with horse that have a forward balance.
Good luck :)