I see three approaches, the first of which addresses the core problem that caused your question, and the second and third directly answer your question.
The easiest and most preferred way to fix the problem is for your DM to use passive perception properly. When your DM learns how passive perception works, you no longer need to change existing mechanisms to take into account a status your DM ignores.
Your DM should use passive perception to see if you notice things unless you are specifically looking for something. This would be used to notice that the guy overhears your bar calls, subtly obscures things in your home, or that your bags are suddenly a little lighter on the market. Passive perception is used to notice things when you are not actively looking for them.
This is different from active perception, where you actively use your mind to find something. This could be that you are trying to figure out where an archer has just shot at your teammate, where the goblin running around in the shadows has run down, or overheard a conversation taking place behind closed doors.
The biggest advantage of passive perception is secrecy. When the DM calls for a roll, as a player you now know that there may be something you have not noticed, and you may be responding to the throw (Meta-Gaming).
You know your DM better than we do, so you need to decide if it is possible to apply the rules properly and how to deal with the situation. Highlighting the ability of the DM to remain secret, however, is a big selling point to prevent meta-games. In addition, compliance with the rules (RAW) of your DM means that you do not need to balance and change your heroics, so you have less work overall.
2. Ban the Observant feat if Passive Perception is not used.
A quick search for camouflaged monsters made me deliver a shadow and an invisible hunter as two decent disguised monsters of varying levels. But I'm sure there are better examples.
A shadow is CR 1/2 and gets +4 for stealth. A small group of these would be a viable encounter for level 2 and would normally allow the use of hiding mechanics to gain advantage. With such a high level of skills, this would not be such a threat. If your DM does not use passive cognition, it is also likely that he will perform free active perceptual tests in combat (which is also not RAW) to spot hidden creatures that have other drawbacks to secret animals.
An invisible stalker is a CR 6 and gets +10 on stealth. The monk's stealth detection would almost equal the stealth ability of an invisible stalker if the Observant talent gave all the perceptions +5. An invisible stalker would, however, lead to short-term work in most Level 2 parties.
If you do not allow Observant to work this way, the maximum perception of characters is level 2 +7 (+5 from 20 WIS and +2 from Proficiency, unless I missed something). The difference is that the character has invested heavily in his limited status points in order to gain such high skill and has sacrificed other benefits to gain that.
As a compromise, this +7 player may not do so much damage (lower STR / DEX / INT / CHR) or may not be as stable as (lower CON). Clerics, Rangers, and Druids have WIS as their spell modifier, and would not sacrifice damage to their maxima this way, and none of these three classes would use spells as the primary source of damage done at level 2.
Also, no other performance gives the player an unconditional bonus to their abilities in this way; They usually augment existing actions with additional skills and give a single statistic point.
3. Carefully let the trick as recommended.
If you allow Observant to be used this way, the level 2 perception is a maximum of 12 (+5 of 20 WIS, +2
If the DM wants to give this player challenging perceptual exams, he has to increase the stealth points of the monster, which makes it much more difficult for all other characters. The same applies to hiding traps or secret communications. They are either trivially light for this character or exceptionally hard to see for the others.