It looks like Matt's facing 5 -10 years in Japanese prison (s).
Best case scenario (below), "large fine and deportation."
Here's a quote from a Quora poster.
"You will be immediately arrested and taken into custody if its airport police you will be taken into immigration detention. These places make American jails look like summer camp.
Cops have their own police cells at the airport and there are no lawyers. Suspects can be held for up to ten days without charging you and then they get another 11-day warrant from a judge. In Japan you are guilty until proven innocent and most likely they will interrogate you until you confess. Once you confess to importing or possessing drugs then getting a conviction is a slam dunk with a judge. You either face real jail time or depending on the mood of the judge, a large fine and summary deportation in handcuffs.
PS do not expect your embassy to bail you out of a Japanese jail if you get arrested here. They will let your family back home know or pass on any messages and that’s about it."
Not sure if prison time in Japan can be reduced with good behaviour, etc, as in many countries.
Matt's no angel, but spending time in a Japan jail/prison would seem to be massive overkill.