This could be one of several things, considering his age and your description:
1. Flea bite hypersensitivity; in those cats that are allergic to flea saliva, even one bite can induce an allergic reaction, skin inflammation and infection. Even indoor cats can get fleas unfortunately, so this is the most likely; you don't often see adult fleas on cats, which further confuses issues.
2. Allergic/contact dermatitis; exposure to something irritating (poison ivy?); inflammation possibly infection of the skin can then occur.
3. Viral infection; certain viral infections, such as herpes, can cause skin lesions
4. Eosinophilic granuloma complex; this is a condition that can affect the skin of younger cats and involves dysfunction of the immune system.
5. Other allergies- airborne or food
I would recommend taking Teddy to your vet this weekend and having him looked at to ensure its nothing serious; its better to get treatment started early rather than wait until things get worse.
Hope this helps! Please let me know if you have any other questions!