Thank you for the additional information. Poor kitty, sounds like he is pretty itchy, and that this bump is a little tender. Since he spends some time outside, there are several possibilities. First would be a bite wound from another kitty, which may be infected. This can be pretty uncomfortable, and often needs antibiotics to clear up. Some need to be drained, but because it is so painful, it is best done by a vet where pain medicines and mild anesthetic drugs can be given as needed. An e-collar is often necessary to keep kitty from trying to lick the wound as this can make it worse and lengthen healing time.
Another possibility, depending on the climate you live in, would be that your kitty was bitten or stung by an insect. Obviously if it is very cold where you live, this is very unlikely. Some bug bites or stings can cause very severe allergic reactions, so keep a close eye on your kitty. If you see any facial swelling, hives (will look like tufts of hair sticking straight up all over), vomiting or diarrhea, your kitty will need to see an emergency vet right away as an allergic reaction is likely.
Also possible would be an illness that is changing the skin and hair coat of your kitty, or maybe his anal glands are full and aren't expressing normally. If the glands are full and uncomfortable, a kitty will sometimes lick and bite at the base of the tail (I know you haven't seen any of this but I mention this possibility to be thorough). Some illnesses to watch for can include diabetes, kidney disease and hyperthyroidism. They are more common in older cats and can drastically change the hydration status and skin and hair coat condition, sometimes resulting in lesions. Yet another possibility is that you kitty has a growth at the base of the tail that has been uncomfortable, and this may or may not be cancerous.
I know that it is New Years Eve, and your kitty will need to be seen.....but as long as he is moderately comfortable, eating and drinking and able to use his litter box without any problems, he can likely wait until tomorrow or maybe friday. If it appears that he is extremely uncomfortable and cannot rest, or you are very worried you can take him to an emergency vet just to be sure. Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Good luck with your handsome man, I hope he feels better soon. Happy New Year, be safe!!!