There are several home remedies for flea bite reactions luckily, so hopefully, your poor dog will be feeling his usual self very soon.
Apple Cider Vinegar
This is a remedy that has been used for years for, amongst many other things, skin problems. It is best to mix it with an equal amount of water and then you can rinse your dog with it after bathing him using a mild shampoo.
Another tip for this method is to keep some of the vinegar/water solution in a little spray bottle. If he starts to scratch, gently part his fur and spray a little straight onto his skin,
This will not only soothe and moisturise, bit will draw toxins out of the skin. All you need to do is grind a cup of oatmeal in a blender and add it to some warm water. Bath your dog with the solution and leave it on for about 20 minutes.
Baking Soda is something else that can be used by just diluting a teaspoon of the powder in warm water and then just dab it onto the itchy patches of his skin.
I hope that helps :-)