Hello, I am sorry to hear about Mojo.

Cats that have fleas don't necessarily itch unless they are allergic.
So if Mojo is the only allergic one in the house then he may be the only one to show symptoms. And it doesn't have to be a huge flea problem. One flea bite a month is enough to drive an allergic cat crazy.
Allergies are the most comon cause of itchy, uncomfortable skin. And flea allergy is the most common allergen.
I'd use flea protection every 28 days. Both Advantage and Frontline work quite well at killing fleas and are less likely to create a skin reaction. You must treat all cats though as if you don't the untreated cats will harbor fleas to attack poor Mojo.
Other allergens are inhaled (like mold spores or pollens) or food allergies.

To treat his allergic reaction to the flea bites (or any allergy) I recommend antihistamines and omega-3 fatty acids. Some antihistamines to try are:

1) Benadryl at 1mg per pound or 1/2 of a 25mg tablet per 10 to 12 pound cat 2 to 3 times daily. You can crush the pill up and hide it in something tasty like his canned cat food or you can give it directly. The alternative is the liquid formula they make for children which is dye and flavor free. There are different concentrations so if you aren't sure about dosing just reply to me and I'll help you figure out the correct dose based on the product's concentration. Make sure whatever you use is Benadryl only as the combination products with decongestants and acetaminophen are toxic to cats.

OR

2) Chlorpheniramine 4mg per cat once or twice daily.

The omega-3 fatty acids will work synergistically with the antihistamine to soothe his itch and as a bonus will improve his hair coat and skin condition in general. I like the products by Derm Caps or 3V. These come in pump form to put on the food or you can puncture the capsules and dribble it on his food.

Another possibility for his discomfort is a condition called hyperesthesia. Hyperesthesia is a poorly understood syndrome which is difficult to diagnose. These cats have pain and hypersensitivity to touch on their back and tails. They sometimes will bite at themselves because they are so uncomfortable. It is a poorly understood disease process with no definitive test for diagnosis and so other problems (such as spinal arthritis or infections or allergic reactions) should be ruled out first. We do know that stress does seem to make this disease process worse or flare more frequently. Sometimes mood altering drugs and steroids can be helpful.

Here are a couple links discussing hyperesthesia syndrome if you are interested: http://www.veterinarypartner.com/Content.plx?P=A&A=1998&S=4&SourceID=62
http://www.cathealth.com/hyperesthesia.htmI
In your fellow case I'd start flea protection and use antihistamines/omega 3's to control the allergic symptoms.
If he continues to be uncomfortable an exam to check for other less likely problems is indicated.