take your time! Five is not mature for arabs. They don't really mature til six, I think.
If you mess too much with forcing him to lower his head, and you end up pulling him either behind the bit, or pulling him back INTO his neck (shortening it), then it will be very hard to undo this habit. First and foremost is ALWAYS . . . Forward! Get him walking freely, energetically forward. Feel of his mouth with the bit (widen your hands a bit until you kind of feel him contact the bit) and follow that feel. Bring your hands back closer together without losing this very soft, following contact. Now, put leg on and ask for more forward and as you feel him step up, you advance your hands forward a bit. Not so much that you lose the contact with his mouth, though. You are "inviting" him to follow the bit forward. If he does, even a wee bit, then give him a wee bit more to say, "yes, that's it" and ride him stepping forward into that open space in front. The more you encourage him to follow the bit forward and downward, rather than asking him to come downward and backward, the more you will preserve his forward.
Again, he is young , so go slowly and correctly and do not pull himn back into a broken S shape, as one sees on SO, SO many Arabs that are pulled back into their own necks.