How do I restart enemy spawn when boss is Destroyed in Unity C# version 5.2

I have a boss that is spawned after a period of time. when it is spawned my other enemies stop spawning. i want my enemies to spawn again after my boss is destroyed. here is my code for it.

Boss Spawn:

 using UnityEngine;
 using System.Collections;
 
 public class BossSpawn : MonoBehaviour {
 
     public GameObject enemy;
   
     public Transform enemySpawn;
     public float SpawnRate;
     private float nextSpawn;
     public GameObject cloneEnemy;
     public float firstSpawn;
     public Transform boss;
     public bool IsSpawned;
     void Start()
     {
         
     }
 
 
     void FixedUpdate()
     {
         if (Time.time > firstSpawn)
         {
 
 
             if (Time.time > nextSpawn)
             {
                 nextSpawn = Time.time + SpawnRate;
                 cloneEnemy = (GameObject)Instantiate(enemy, enemySpawn.position, enemySpawn.rotation);
                 gameObject.SetActive(true);
             }
            
         }  
     }
     void Update()
     {
         
         if(boss == null)
         {
             IsSpawned = false;
         }
        else
         {
             IsSpawned = true;
         }
 
     }
     
 }

Boss Behavior (code that applies):

 public GameObject bos;
 
 void OnDestroy()
     {
         bos.GetComponent<BossSpawn>().boss = null; 
     }

and Enemy Spawn:

 using UnityEngine;
 using System.Collections;
 
 public class enemySpawner : MonoBehaviour
 {
     public GameObject enemy;
     public Transform enemySpawn;
     public float SpawnRate;
     private float nextSpawn;
     public GameObject cloneEnemy;
     public bool bossIsSpawned;
     public Transform bossSpawn;
     void Start ()
     {
        
     }
     
     
     void FixedUpdate ()
     {
         if ( Time.time > nextSpawn && bossIsSpawned == false)
         {
             nextSpawn = Time.time + SpawnRate;
             cloneEnemy = (GameObject)Instantiate(enemy, enemySpawn.position, enemySpawn.rotation);
         }
         if (bossSpawn.GetComponent<BossSpawn>().IsSpawned == true)
         {
             bossIsSpawned = true;
             
         }
         if(bossSpawn.GetComponent<BossSpawn>().IsSpawned == false)
         {
             bossIsSpawned = false;
         }
     }
 }

Enemy Spawn does work. my main problem is getting IsSpawned = false when my boss is destroyed.

Thanks for the help! mnmwert