Did you try putting your SIM card back into your iPhone, then going to Settings > Messages and turning off iMessage over there? Make sure you have an internet connection too so it can "broadcast" your new status to Apple HQ. This is what I do when I'm abroad and don't want to dataroam. It's possible you already tried that and it won't make a difference - I don't use group messaging a lot.
This would also work if a friend lent him their phone.
Of course, he'd have to do a backup > wipe > setup phone with his iCloud account & SIM > change settings > wipe > restore from backup... but, honestly, this is the process I had to do and it took about 5-10 mins tops.
I had this same exact problem, and this was by far the easiest solution. Just need to have a friend willing to you let you manhandle their phone for a little while.
Yeah, all that OP has to do is go into the settings on the iPhone and either unlink the phone number or disable iMessage entirely and it'll go back to normal. It just required some forethought other than simply wiping the phone. I don't understand what the issue is; the feature is there so that you can be iMessaged if your phone happens to be turned off for a few days.
If the phone number was de-linked immediately what would the use in that be? It expires after 10 days, anyway.
