Recurring infection

My cat is 13 now, and he was perfectly healthy until about a year-2 years ago. He has kitty gingivitis, and the infection spread to his nose, which he tends to rub raw. His fur is matted, his paws are actually a brownish colour from the blood that comes off his nose and he can only eat his wet food again, he doesn't touch the dry food, and he's lost weight again.

I took him to the vet about a month ago, and he got 3 injections as well as antibiotics to be taken morning and night, which my father gave him. He started doing a lot better when on the antibiotics, but the course is finished now and the infection has returned. As it keeps doing. 

He's been to the vet numerous times before that, getting injections and antibiotics and it just keeps returning. I don't know what to do anymore. I know somebody who sells cannabis oil, and I know that tends to help people... I just don't know if I can use it for a cat? 

The vet also told me that if the infection returns again, I have to have him tested for feline aids. So that scares me.

Please, if there is anything at all anyone knows of that I can do, let me know. I love this cat so much it hurts. I'll be heartbroken if something happens to him. 

Thanks all.

Is it gingivitis, or stomatitis?  With gingivitis, usually a dental will help with that.  Stomatitis is a different story.  Here are a couple of threads about it:

http://www.thecatsite.com/t/296414/treatment-for-stomatitis

http://www.thecatsite.com/t/318119/stomatitis

There are many, many more threads on stomatitis.  Just do a search from the top of the page to find them.  This does sound like what you are now dealing with.

Thanks a million. 

I gave it a look, and Googled it too. It does sound very similar to what my boy has. What confuses me is that the vet straight up said it's gingivitis... but we'll be taking him to the vet this Saturday again, for a checkup and then the testing, if need be.