As you lose muscle the ones left have to work harder to hold up all the weight. Have you tried any of the Heet rubs or linaments or the icy cold stuff? If it gets real bad there are directions on the forum for a home made neck brace.
My dad had neck and back pain from the stress of staying upright and holding his head up (as Al said from losing muscle mass and strength). Eventually he lost all muscle function in his neck and was unable to hold his head up at all. Before the ALS diagnosis, doctors were reluctant to have him use a neck brace, as they thought that would cause more muscle weakness (use it or lose it). After ALS was diagnosed, it was clear he would lose the muscle anyway, so a neck brace was prescribed. Neither of the commercial braces he tried worked very well, and we had to amend them with foam and fleece at the pressure points. I think it is definitely worth trying the home made neck brace, as it is specifically geared towards ALS needs. My dad passed away before I had a chance to make one for him, so I can't give you personal experience with it unfortunately. The problem with his neck was troublesome, made it harder to eat, see where he was going, etc. so its definitely worth coming up with a viable solution that works for you before it gets to the extreme. Good luck with it.
Somebody, I'm pretty sure it was landscape, started a thread one time showing instructions about how to make a homemade neck brace. From previous posts on this subject, I get the impression that the manufactured braces are not quite right for PALS.
The full spine and head brace worked better than the one that went between his chest and chin, but it was very cumbersome to put on. He had practically no muscle left in his back or neck, so needed more support.