Hi again, Denise and thanks for your helpful reply.
Also, thanks for your patience, as I had a lot to say. : )
There are various possibilities of what these sores/bald spots could be. One is ringworm, a fungal skin infection not caused by a worm at all. It usually causes hairloss as the infection makes the base of the hair brittle,and it breaks off. Sometimes ringworm is itchy and sometimes it's not. The only way to know for sure if this is ringworm, is for the vet to look at the skin under a special light called a 'Woods lamp'. Ringworm is quite contagious between cats and also between cats and humans, so until you know for sure what is causing his symptoms, try to wear gloves when you touch these areas. Here is more information about ringworm in cats: Ringworm
The sore on his neck may be caused by a bot worm. The bot fly lays eggs and the larvae burrow under the skin, leaving a tiny 'breathing hole' about the size of a pencil point. This condition is called 'cuterebra'. Check to see if you notice a tiny hole like this in any of the sores. You also might see the tiny white worm sticking it's head out of the hole, if this is the condition he has. The tiny worm is full of dangerous bacteria, so you wouldn't want to squash it in the skin. It would be best to have the vet remove it carefully. Here is more information: Cuterebra
Because Leo doesn't spend any time outside, the chances are lower that this could be cuterebra.
Another possibility is allergies. This could include inhalant, flea, contact or food allergies; the fact the you just started a new food could be related, but since you mentioned he also hadn't eaten for 3 days, this is of concern. When a cat doesn't eat properly or at all for even a short period of time, he can risk developing a serious liver condition called hepatic lipidosis or 'fatty liver'. Here is more information about allergies in cats: Allergies
Try not to use hydrogen peroxide to clean these areas, because it can be rough on the skin tissue. It would be better to use diluted Betadine (add water until it's like the color of weak tea).
At this time, it would be advisable to have Leo seen in person by a vet, for evaluation, diagnosis and treatment. Make sure to mention he hasn't been eating as usual and you recently changed food. A routine senior blood test would also be a good idea, as older cats are more prone to develop medical conditions like diabetes, hyperthyroidism, liver and kidney issues. These could also affect his appetite.
I hope all will be well with Leo once he is diagnosed and prescribed treatment by the vet.