[@Vocab]Well, you're going to get some more of hardass!Clock:

If he had a relative who lived outside Japan and could provide him with money, then why not just use said money to buy a plane ticket and bring Mastu to them?

Part time jobs and government assistance might help him with living expenses and collecting otaku stuff, but not for blackmarket weapons. Japan isn't America where you can just go into a gun show, wave some money around and go home with an AK-47, no questions asked. I presume you'd first have to go meet a criminal contact which is hard to say the least who would be connected to the Yakuza or some other gang. Depending on how young Mastu first met them, they could have easily lured him into a trap and sold him into slavery.

Japan also only produces guns for domestic military usage meaning that you'll only find JDSF weapons in a military armory or base and anyone trying to sell them would charge a HUGE price assuming there is even anyone in the JDSF who'd sell to a kid. Thus leaves the only other option of importing weapons, but again Japan is tight on smuggling meaning that the only smugglers you'd find are the ones mentioned above you'd be connected to a criminal web in some way and would also probably not sell to a kid. Same story with the ammunition and where would he even practice shooting? Gun ranges are not common place in Japan.

If you want my opinion, all of this can easily be solved by simply taking that part out of the CS.

As for the fighting, you can just say "experienced" which implies he's been through the mill but doesn't suggest he's won all of them nor has he lost all of them. The bladed weapon would also probably be a big maybe as depending on who he pissed off and shanked, they'd come back armed and with a vengeance knowing that Mastu has a weapon of his own. Again, I suggest you just remove that from the CS.

Also, the only people who'd actually attack someone for insulting them are egotistical punks. If Mastu managed to get something like an entire train full of people angry at him, he'd probably get in some form of trouble and perhaps jailed for repeated offense if he did it enough.

Phychiatrists are trained to be not fooled by people who wear masks, sure he might be able to fool a newer one, but if he came in contact with one who knew what they were doing, his mask would crumble without a doubt. That won't change the fact that he'd be under some kind of survalence which would make him getting weapons even harder if it was even possible.