The first thing you have to remember, is that you might be the absolute authority in the world, but that doesn't mean you have to know everything beforehand. It also means the characters only know, what they experience. The character somehow woke up with super-vision instead of normal Blindsight. They might not thought about it, but that is strange. And strange is the trade of adventurers, so make them realize that it is strange. The best way is to frame it like you planned it all along (even if you had no clue or idea about it. It is an ancient DM secret).
Talk to your player about the mechanical effect, but frame it as an opportunity for adventure. ("I feel that your constant 360 wall-hack vision is too powerful, so I will have to change it. Are you up to a cool sotry about it?"). After all a major change in capabilities like this should be a big motivation for the characters, and thus, adventure hook.
DnD is magical place where anything can happen. Use that. Give out ominous clues. Keep them as indistinct as you can (the classics like sour milk, two headed lambs, strange cold winds etc. might work best.) Then, after a night of not so sound sleep, the ability is just gone. No explanation, no saves, no nothing. Let the party try to figure out a cure (go in a dungeon, find a magic item, beat up some wildlife, find a local cure, find the spirit who stole it, etc.) If you feel like somebody playing a blind character for a while would be bad for the game, make a partial cure available.
Maybe the village they are in has a huge catacomb system, where they use ointments of Blindsight. Maybe they find a pendant on an adventurer in a dark tomb, who suspiciously lacks torches. Etc.
So after a session of him not having his sight, after they finish the ritual/find the doodad/kill the boss/free the high priest give him back the normal Blindsight, preferably with the opportunity to use the super-version at some cost (Con damage/Wis damage maybe? I am not the best on DnD rules). That way the he keeps his cool ability, there remains some consistency, but using and abusing it becomes prohibitive because of the cost.