Sorry for your loss. There are many tools that support bitcoin and many services that store it.
Check e-mails for services like blockchain.info,coinbase.com, poloniex, bittrex, etc. Anything with words cryptocurrency and bitcoin in it, he might have invested in different platforms and cryptoexchanges, make sure to search for everything.
If you believe his wallet is on the HDD, check for wallet.dat file on the computer. Also, Electrum is a popular bitcoin storage tool, doesn't label its wallets with a dat extension. You would have to check for that in the directories. If you need any more advice, let us know. Good luck.