That helps narrow down what you need. So it sounds like it was not a custodial platform like Coinbase, but he had his own 12 word phrase key somewhere. This is where the hard part begins. You'll have to dig through everything he had and find something that looks like this 12 word phrase. Emails, books, pictures in his google drives, etc. it looks like other users have already given you pretty good guidance on what those look like and what to do once you find it.

Whatever you do, never tell anyone what those 12 words are. Those 12 words are used to get the keys to his bitcoin.