Shit. He upgraded to those hack-job "5.02 M33" variants that some fools cobbled together, and I warned against a few days ago? There are two, that I know of, and one of them expressly disables ISO loading, as I understand it.
I can't say I know about the ins and outs of OFW/CFW flashing, if some methods (regular installer, DC, Pandora) perform "more complete" flashes or not. But if I had to guess, then that non-Dark-Alex 5.02 M33 flasher that your brother used broke something that the flashers we've used so far isn't fixing.
Have you tried using a Pandora to re-flash your PSP? Perhaps that'll catch something DC missed? We're in unknown territory for me here. But I'll try to see if I can scrape up some more answers somewhere, in the mean time.
Sorry for the delayed response. I'm fielding loads of Server 500 errors right now...
Edit - And yeah, when I have compatibility issues with ISOs or PSOne games, it's usually a black screen hang followed by an auto-shut off half a minute or so later.
Edit 2 - Boot into the recovery console and go through the menus one by one, looking for anything that might be off. Sometimes flashing a lot of firmware updates can dick around with your ISO loading settings. I don't think so, but it's possible that your ISO issues are related to that. Make sure you're set to use the M33 UMD ISO loader, not the Sony one (or any other options)...