I have 15 month old Daisy, and her momma Lily. Daisy has yet to make as mature of a hee haw as her momma. Usually she just sounds like an obscene phone caller or like she's trying to hock up a lung. Momma, however, can really let her rip, and it seems to me like as long as Lily is doing the job, why bother?!
The main times when the bray is: when they see me or my husband coming, when someone pulls in the driveway, when they want to be let in for their grain, and when the rural fire department siren goes off. That sounds like a lot, but really it's only a few times a day. In the depths of the summer when they have lots of grass and the fire alarm isn't going off, I don't hear from them for days.
So give him time. He'll figure it out, then you'll be sorry he did!!
Jayne