I'm sorry to hear your cat is experiencing these problems.
No, in general, the yeast infection in his ears shouldn't be related to your cat's hairloss on his left side, however, once there is any kind of infection in the body, whether it is bacterial, viral or fungal (yeast), it can cause side effects like hairloss, as the immune system is compromised.
Very often, hairloss only is certain areas, may be caused by external parasites like fleas or mites, and cats sensitive to flea saliva may exhibit a reaction all over the body, or on one side, as you've seen, from the bite of just one flea. Even if you don't see fleas on your cat, just the presence of one flea at one time, even a while ago, can cause a reaction in your cat's skin.
Other allergies, in addition to flea allergies, like sensitivities to ingredients in his food, contact allergies with something he's laying on or rubbing against, perhaps with only his left side, can also cause hairloss.
Oddly enough, ringworm, a fungal infection, causes hairloss, but usually the skin under affected area will appear like a red, circular pattern, and may be scaly. The yeast infection in his ears, may be related to the fungal infection ringworm, if the skin under the areas of hair loss resemble my description.
Being on a quality flea preventative cannot be a bad suggestion at this time, and your vet is trying to cover all bases. There is also a psychological or emotional cause of hairloss called psychogenic alopecia, and you can read more about it here:
I've copied the most relevant part of the article for you, below:
This disease is diagnosed in case of a self-inflicted alopecia, when all allergic/parasitic causes have been excluded and when the cat has some other behavioural problem. Obsessive compulsive disorders (OCD) derive from species-typical behaviours, such as vocalization, locomotion, and grooming. Behaviours derived from grooming include acral lick dermatitis in the dog and self-licking and hair chewing/pulling in the cat (1). The main cause of OCD is a reactive abnormal behaviour, as an expression of a conflict induced by inappropriate environment or management. If the conflict persists, the behaviour might become repetitive and develop into a stereotypy, be performed out of that context, be generalized to any unfavourable situation or be displayed without any identifiable eliciting stimulus. Stress-induced release of opioid peptides is able to sensitize normal dopaminergic neuronal pathways, which become more easily excited, so that any further arousal is then channeled through these pathways and result in that particular behaviour (2). Endorphins (opioid peptides) might be important in the onset of the behaviour and dopamine may be more involved with the maintenance of the behaviour.
There seems to be a breed predisposition in Siamese, Burmese, Himalayan and Abyssinian cats (3). The clinical appearance is the result of chronic chewing, licking and hair pulling by the cat, which results in alopecia on the middle of the back, perineal, genital areas, medial thighs, ventral abdomen, front legs, shoulder and feet. The skin surface usually shows no lesion, unless traumatized by the tongue. The excessive licking can be accompanied by other symptoms, such as a rippling motion of the skin over the back, periodic unexplained states of agitation, a glazed appearance of the eyes with semidilated fixed pupils. The microscopic examination of the hair (broken tips), the application of an Elizabethan collar and the exclusion of all pruritic causes of self-licking (allergies, parasites) lead to the diagnosis. Therapy is based on identification and removal of the stress cause, supported by the use of psychotropic drugs.
1. Antianxiety drugs: amitriptyline (5 mg PO BID), diazepam (1-2 mg PO BID or SID)
2. Drugs that specifically interfere with the neurologic pathways involved in stereotypies: opiate-receptor blockers and dopamine antagonists (4):
a. opiate-receptor blockers: naloxone (1 mg/kg SC once) (naltrexone, nalmefene, diprenorphine) competitively inhibit ß-endorphin receptors, and seem to be more effective in short-term stereotypies (<1 year)
b. dopamine antagonist haloperidol (1 mg/kg PO SID initially, then taper), yields a better effect if the stereotypy exists for a longer period (> 1 year).
3. Drugs that selectively inhibit the serotonin re-uptake: fluoxetine (1 mg/kg SID), clomipramine (1,25-2,5 mg PO SID), imipramine."
Right now, I feel your vet is treating your cat's condition is the most appropriate manner. Wait a while to see some positive results, and hopefully the treatments will be successful. If not, your vet will have to do some bloodwork, and a skin scraping (painless) for analysis, would be a good idea.
I hope all will be well with your furry fella!