I am so sorry your little mate has been diagnosed with asthma. It can be a frustrating condition to manage. It is wonderful that you are considering the long term side effects of medications and are interested in what will be best for your cat in the long term.
Feline asthma is a lower respiratory disorder that consists to inflammation caused by an allergic reaction. It leads to constriction of the airway and limits air flow. Unfortunately, there is no cure for feline asthma, it is about managing the disease using environmental modification and medical therapy.
Initially treating with oral steroids is required to stabilise the inflammation occurring in the airways and monitor for response. Once he is stable on this medication it would be a good idea to chat to your vet about the option of transitioning onto inhaled steroid medication designed specifically for cats. This does come at an increased cost compared to oral steroids; however, it has fewer systemic side effects so would be ideal in the long term, particularly considering your little darling is so young.
Another medication that can be used in conjunction with steroids are bronchodilators. Once again these come in oral or inhaled forms. These can be useful for intermittent flare ups, for example, when your cat starts to wheeze or have noisy breathing.
A really important component to managing feline asthma is eliminating allergens/irritants form the environment as many cats will improve with better air quality. I would suggests eliminating or reducing aerosols, house hold chemicals, perfumed products, smoke and dust from your household where possible. It would also be worth trialling sand or plain clay cat litter if not using these already. Try cleaning your carpets, furnishings, bedding, drapes and heating units/ducts. Change air filters regularly and use a vacuum with a HEPA filter. Using an air purifier can also help improve the overall air quality. These suggestions may sound extreme, but can certainly result in better control of the asthma for your little cat.
If you are finding that you are giving the medications as prescribed and your little mate is not responding, it is possible something else could be going on, or he may have a secondary bacterial infection in the mix. Making an appointment with your vet for another examination and potentially further work up would be a good idea.
As far as the effect of heat, your cat is at greater risk of overheating and respiratory distress due to his decreased ability to control his temperature. Cats rely on dissipating heat via their airways. If their airways are constricted or coated in mucus, as they can be with feline asthma, this mechanism is compromised. It is important for him to be kept in a cool environment on hot days. Perhaps have a discussion with your vet about your home environment to come up with a more economical way of keeping him cool during the hot weather.
I hope this has helped you and your darling boy! I wish you both all the best. Keep in touch and let us know how you get on.