No. His death means nothing he spilled comes in at trial because he can’t be cross-examined. The evidence he gave can be used for investigative purposes to try to develop other sources, but the particular evidence he gave will have to be developed another way, which it very possibly can’t be, in order to be used at trial.
Well if they bothered to get a sworn affidavit it could be used as evidence but only on things he knows directly and then it is limited because as you say, they can’t cross examine him. That is the very reason witnesses are killed.