I'm sorry to hear Monty is not himself recently. It sounds as though you are providing him with a great quality of life and doing all of the right things.
Sadly, most pet mice only have a lifespan of 1 to 2 years so little Monty is doing well, and hopefully will continue to do so. However, in their old age, this is when we typically see things starting to go wrong.
There are a few things that can cause red sores, over-grooming and alopecia (baldness).
1. Mange. Older mice are more prone to mange/mites. Though it may seem odd that they could contract them if not in contact with others, all mice actually have a small number of mites on their skin naturally and sometimes they proliferate in older or unwell animals. The vet can prescribe some medication, usually a topical spot-on for this. The diagnosis is quite easy and consists of taking a gentle scraping and looking at it under the microscope. His cage and environment would need disinfecting after treatment. Lice are also a possibility, though would be visible to the naked eye.
2. A skin infection such as a fungal or bacterial infection is possible. A vet could take a swab/pluck of the affected area to analyse. Ringworm (fungal) infections tend to be dry/scaly while bacterial infections are often wet/oozing, although this is just a general rule. Medicine would depend on which type of infection he had. We also want to minimise any over-grooming or itching he is doing, and a medicated wash may be prescribed for this.
3. Sadly, cancer is a possibility, particularly a lymphoma. These lesions can grow quickly and spread fast. A biopsy would diagnose it. Unfortunately, as it can be aggressive and many animals will be put to sleep once the condition is advanced.
4. Something else such as an endocrine (hormonal disorder), allergic reaction (to e.g. new food or bedding) or nutritional deficiency are all also possible, although would be less likely at his age.
Claws can be gently trimmed to limit self trauma.
I would suggest a trip to the local vet who will give Monty an all over check in his old age and may prescribe some medication.
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