The biggest mistake people make here is panicking and hiring someone who promises to "recover" the keys - a lot of those are scams. Don't give anyone access to accounts or pay upfront recovery fees without heavy vetting. Beyond the obvious (searching his email, cloud storage, password managers, physical documents), reach out to his financial advisor, accountant, or attorney - sometimes people mention crypto holdings in estate planning documents or store seed phrases in safe deposit boxes that you might not have checked yet.
Here's something less obvious: contact the exchanges or platforms where he might have held these assets directly. If he had accounts on Coinbase, Kraken, Gemini, or similar services, the exchange itself can sometimes help with account recovery if you have a death certificate and proof of heirship. Same with NFT platforms - OpenSea and others have had some success with inheritance claims when proper documentation is provided. This won't work if he kept everything in a self-custody hardware wallet with no recovery phrase, but it's worth exploring before giving up.
On the tax side, yes, it's treated differently than regular inheritance in some ways. Most states don't tax inherited assets as income to you, but crypto and NFTs have a "step-up in basis" when inherited - meaning your tax basis is their fair market value on the date of death, not what he originally paid. When you eventually sell them, you'd only owe capital gains on the appreciation after that date. The IRS does want this reported, so you'll likely need to file an estate tax return if the total estate exceeds the threshold, and document the valuation of those digital assets as of his death date. A tax professional who handles crypto specifically would be worth consulting here.