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Interesting, you could get into a myriad of problems later on though. I don't know your particular setup, but you could adjust the height and with of the collider, when you need him to go through tiny gaps. 

I'm trying to play it again now, but it's better to adjust now, and do it properly, because it will give you headaches later on, when you work on bigger games. Maybe this can help you?

Try if it works for your Project.

It adjust your characters capsule collider by half or whatever you put into the inspector, that should give you an idea, how you could make it work to get through tiny spaces.
 

using UnityEngine;  
public class CharacterColliderControl : MonoBehaviour
{     
[SerializeField] private CapsuleCollider characterCollider;     
[SerializeField] private float normalHeight = 2f;     
[SerializeField] private float crouchHeight = 1f;     
[SerializeField] private Vector3 normalCenter;     
[SerializeField] private Vector3 crouchCenter;      
private bool isCrouching = false;      
void Start()     
{         
if (characterCollider == null)         
{             
characterCollider = GetComponent<capsulecollider>();  
}       
if (normalCenter == Vector3.zero) normalCenter = characterCollider.center;         
if (crouchCenter == Vector3.zero) crouchCenter = normalCenter / 2;    
}      
void Update()     
{         
if (Input.GetKeyDown(KeyCode.C))         
{             
isCrouching = !isCrouching;              
if (isCrouching)             
{                 
characterCollider.height = crouchHeight;
characterCollider.center = crouchCenter;
}            
else             
{                 
characterCollider.height = normalHeight;
characterCollider.center = normalCenter;   
}         
}     
} 
}