He can't do it quickly enough to use the money for the cremation. If there isn't a will or some other document that says who should get it, then this is probably going to have to go through probate and be handled according to the intestate laws of the state where he lived. First, whatever he owed has to be paid; if it's more than $900, then all $900 has to go to whomever he owed; if he owed less than $900, then whatever he owed has to be paid in full, and only whatever is left after that can be used for the cremation.
Normally, if a person isn't married, then whatever is left (after paying what he owed) is supposed to be split equally by all the children, so if some of them don't want anything, then they will have to sign paperwork disclaiming their rights to inherit. Or maybe it would be faster not to bother with that, and have equal amounts of the money distributed to each son, and then have them each contribute that money towards paying the cremation expenses. But, again, if he owed more than $900, then it's a moot point, because the credit union can't give any of the money to the son; it must go to paying what the father owed.