I would actually say that most of the advices here actually encourage the hoarding behavior.
OP, if you are still here, just take inventory of all the things in his warehouse then look up the usual agreed prices on those collectibles and then sell them for 40-50% of its "value". sell EVERY SINGLE THING and sell every single thing FAST.
doing anything more like >>6891496
is suggesting would just encourage him again to hoard toys and crap after all of this is done.
don't worry about how much value of the things he has collected. if you are agonizing how much money you can get from a toy or card, you are falling for the same trap as him.
for real, I sold my friend's basketball cards (a lot of them are the rare gold jordan cards) which are around as usual 300-1000 USD according to wizard, I sold them around 150-500 USD, and from those jordan cards alone, we got around 30000 USD. the rest of the cards were sold around 1-30 USD and we made enough to cover his surgery.