I see three lines of action, the first one dealing with the main issue that sparked your question, the second and third responding directly to your question.
The easiest way to solve the problem, and the one I prefer, is that your DM uses passive perception correctly. If your DM learns how passive perception works, you no longer need to modify the existing mechanisms to take into account a statistic that your DM chooses to ignore.
Your DM should use a passive perception to see if you notice things when you are not specifically looking for something. We would use it to notice the guy who listens to your conversation at the bar, things that are slightly out of place in your home, or pockets that are a bit lighter in the market. Passive perception is used to notice things when you are not actively looking for them.
This is different from the active perception where you actively use your senses to find something. This could be to try to locate where an archer just shot your teammate, to find where the goblin sneaking into the shadows escaped, or listen to a conversation taking place behind closed doors.
The biggest advantage of using passive perception is the secret. If the DM requests a result, you, as a player, now know that there is potentially something that you have not noticed and that you are now potentially reacting to the result (meta-game).
You know your DM better than we do, so you have to decide whether good rule enforcement is achievable and how to approach the situation, but stressing the DM's ability to remain secret is an important selling point for preventing metagame. In addition, playing your DM in accordance with the written rules (RAW) means that you do not have to balance the house brews and change the prowess.
2. Disable the Observant exploit if passive perception is not used.
A quick search of stealth monsters gave me a shadow and an invisible tracker in two stealthy decent monsters of different levels. I'm sure there are better examples, though.
A shadow is CR 1/2 and gains +4 stealth. A small group of these would be a viable meeting for level 2 and would normally allow for masking mechanisms to gain benefits. Against such a high level of competence, this would not be such a threat. If your DM does not use passive perception, it is also likely that it can perform free active perception checks in combat (which is also not RAW) to spot hidden creatures. , which further disadvantages stealthy creatures.
An invisible stalker is a CR 6 and gains +10 in Stealth. The monk's stealth detection would almost match the stealth skill of the invisible stalker if the Observant feat conferred a +5 bonus on all perceptions. However, an invisible stalker would do a quick job at most level 2 parties.
If you do not allow Observant to function in this manner, the maximum perception of a Level 2 character might be equal to +7 (+5 from 20 WIS and +2 from Proficiency, to unless I have forgotten something). The difference here is that the character has heavily invested his limited stats points to get such a high skill and sacrificed other benefits to get it.
In return, this player +7 may not suffer as much damage (STR / DEX / INT / CHR lower) or will not be as robust as (lower CON). Clerics, rangers, and druids use WIS as a spell casting modifier and would not "sacrifice" the damage done to the maximum perception of this kind, and none of these three classes would use the WIS. spells as the main source of their damage at level 2.
In addition, no other feat gives the player a bonus to unconditional skills in this manner; they usually add additional capacity to existing actions and give a single statistic point.
3. Carefully allow the exploit as suggested.
If you allow the use of Observant in this way, the maximum perception of level 2 can be 12 (+5 from 20 WIS, +2
If the DM wants to give this player difficult perception checks, he will have to increase the monster's stealth scores, which makes the task even harder for all the other characters. The same goes for hiding traps or having secret communications. they will be either trivially easy for this character, or exceptionally difficult to perceive for others.