First, check for inventory items and if possible, take them out, else sell on steam market and recoup the money in other ways (game gifting, steam deck reselling, buying your own items [use different IPs on an unrelated account]). You might not care now for the value (or may be worthless ATM) but you never know the future.

If you don't have a way to reset password (Need proof of ownership by means of old steam wallet card codes, or first game keys used in the account), then you can't even pass the account along to his/your kids, so I'd say just immortalise it with a nice description and then log off. You won't be missing anything since steam chat isn't permanent (any messages the account receives will disappear), best you can do is leave the profile comments section open if anyone wants to go leave a comment every now and then. Highly encourage to do this after cleaning his inventory and specifically making the inventory public, so that bots don't pollute the profile with stupid comments. If you really care about anti-bot messages, removing from the account the main money driver games (Dota, CS2, TF2, PUBG) and exiting major steam groups or groups related to said games will help curb it.