That player specialized in perception, so why not let him do his stuff and see everything? In fact, you should consider that a huge bonus for you. He did not choose some wacky combo that lets him do crazy amount of dmg. He took a defense skill and raise it above the norm. Kuddo on him for that. But maybe your problem comes from some missunderstanding of the skill...
Perception is great to spot obvious (and not so obvious) ennemy. Be it an ennemy in hiding (or invisible), listening/evesdropping a conversation or just spotting the incongruous thing once in a while (an orc breathing and lying in ambush behind a secret door...) But it won't let you find a trap, or a secret door. You might know that something is amiss, but you'll have to get an other skill for that.
Investigation is the key to your problem. You will find that this skill is even more important. It lets you find the traps, the secret doors and all the other yaddi yadda related from searching a room up to perusing old books for clues about an astrological event.
IF you feel generous, you might let perception help you find the position of the secret door once in a while, especialy if it helps your player make some progress in the story (hey guys, our torch flickered for no reason, there must be a secret door nearby) or (the secret door must be behind that bookshelf, see the marks on the floor...) but the perception skill won't let you open it. You have to use investigation to find the way to open it nor will it let you disable a trap (Hey guys! These holes in these walls are strangely suspicious, must be a trap. But how is this trap triggered... I really don't know.)
Yes, with such a high perception, your dreams of ambushing them are, for all intent and purpose, over. In fact, as soon as someone gets 16 or above in passive perception, you're toast on that. In a sense, the observant feat is somewhat of an overkill in that area. The real question is if the player has both perception and investigation above 20.This would mean 18 in both intelligence and wisdom...(or a rogue with both skills in expertise and a 14 in both stats). Now your in a bit of a trouble but remember that the observant feat does not increase the skill as whole but only the passive part. In fact, the observant feat raise only your passive skill, not your active skill use. The said rogue mentionned above would still make an active roll at +6, not +11.
With such a character you'll have to use other tactics like reinforcements, waves of monsters, boss fights and whatever means are necessary to further your adventures. Seeing reinforcement in the middle of fight might be done with disadvantage (negating the passive bonus). Allowing some assassins to creep on your group and achieve some minor surprise effects.
In resumé, remember this. Knowing about something does nothing to counter it. You still have to do something with the information gained.