I would say he hasn't forgotten his training as much as he isn't as trained as you thing he is.
And I would also suggest maybe looking into whether or not his feed may be adding to the problem.
Does he get turn out time?
You need to do all of the work with him in the outdoors that you have done indoors also.
A martingale will only keep his head down, but will probably add bucking to his repertoire too, and there will be no way to bring his head up with one.
Like it or not, this horse isn't trained well enough for you to be riding him, as you have left a major hole in his training I think.
Teeth, saddle, other internal problems causing pain? All might have something to do with this, but bottom line is? You need to stay off of this horse until you do more work outside.
Horses make me a better person.