I am sorry to hear that your kitty Roo has a swollen, bottom lower lip. I believe what he has is a rodent ulcer.
See this link for a picture of a rodent ulcer on the lower lip of a cat: http://ww2.justanswer.com/uploads/lboyer/2011-07-27_095304_eosingran.jpg
Rodent ulcers are neither ulcers nor related to rodents. A rodent ulcer is an allergic reaction to something and they can look like a semi-firm swelling of the lip or they may ulcerate and become infected secondarily making them look worse. They are not contagious. They look horrible but many cats don't seem to find them painful and they don't usually interfere with eating or drinking unless they get really big. That means that although this looks terrible to you it is not a true emergency.
The allergic reaction may be to plastic bowls, so I recommend feeding in glass or stainless steel bowls.
These can also be related to a food allergy.
Fish allergies are a common cause. Try to remember if he was given a different flavor of food or treats lately and stay away from that one.
Those things will help stop another from forming and may decrease the size of the one he has now.
Treatment of the current one usually requires oral antibiotics and a steroid injection. So he does need to see a veterinarian. But as long as he is eating well you can have him seen by his veterinarian when convenient this week. Benadryl (diphenhydramine) is great for inhaled allergies, but is not very effective for rodent ulcers.
If it is a continual problem for him then it may be advisable to try feeding a hypoallergenic diet long term to prevent the lesions from coming back.
Rarely, lesions like this can be related to cancer like squamous cell carcinoma or a fibrosarcoma but those are more common in the mouth, either on the hard palate, on the gums themselves or on the tonsils. I have never seen one on the lip, that's how rare they are in that location, so odds are it is more of a benign lesion such as a rodent ulcer.
Let me know if you have any further questions.