If someone swallows his saliva that did not come out of his mouth, there is no blame on him, even if the saliva collects on the mouth guard or on the toothbrush and the like, so long as it is still in the mouth and he did not take it out.
As for saliva that comes out of the mouth, such as if it comes out on the toothbrush or the mouth guard and the like, then the toothbrush is put back in the mouth with the saliva on it, if he swallows this saliva that is on the toothbrush, the fast is broken by that, if the saliva on it is a large amount and clearly visible. As for a small amount, that does not break the fast.
Ibn Qudamah (may Allah have mercy on him) said:
What cannot be avoided, such as swallowing saliva, does not break the fast, because avoiding that is very difficult. Similar to that is dust in the street or particles of flour when sifting it. If he lets it accumulate in the mouth, then swallows it deliberately, that does not break the fast, because it reaches his stomach directly from the place where it was. The same applies if he does not let it accumulate... because saliva does not break the fast if he does not let it accumulate in his mouth, even if he deliberately swallows it, and the same applies if he does let it accumulate (that is, it still does not break the fast).
If his saliva comes out onto his garment or between his fingers or between his lips, then he takes it back and swallows it, or he swallows the saliva of someone else, that does break the fast.
End quote from Al-Mughni, 4/354.
And he (may Allah have mercy on him) said:
If he leaves a pebble or a coin in his mouth, then takes it out and there is some wetness of saliva on it, then he puts it back in his mouth, then it depends: if there is a lot of saliva on it and he swallows it, then he has broken his fast.
But if there is only a small amount, the fast is not broken by swallowing his saliva... because it is unlikely that this wetness could be separated from the pebble and enter his throat, so it does not break the fast, like rinsing out the mouth, or cleaning the teeth with a fresh and wetted siwak.
That is supported by the hadith which speaks of a man sucking on his wife’s tongue. If he brings his tongue back into his mouth with wetness (from her saliva) on it and swallows this saliva, it does not break his fast.
End quote from Al-Mughni, 4/355.
Based on that:
What appears to be the case is that whatever saliva gets on to the mouth guard, a little of it remains when the mouth guard is taken out, then when it is put back into the mouth and the person swallows the saliva that is left on it, it does not appear to be a large amount that would break the fast.
Despite that, he should take precautions to guard his fast. So if he takes the mouth guard out of his mouth, he should dry it with a tissue and the like before putting it back into his mouth. But if he forgets that sometimes, then we hope that there will be no blame on him.
And Allah knows best.