I was just about to edit my post, but you replied back first
Regardless of whether it's rot or biting, the treatment is the same.
And I didn't realise I wasn't supposed to reply??
I didn't see any indication the the OP has resolved the isses. The OP hasn't posted back since she said:
I can get him a bigger tank no problem, but is this going to fix whatever is going on with him? Why was he fine for months and now all the sudden so sick.
Therefore she still had questions, so I gave my opinion. And to answer the above, depending on the cause (rot or biting) a bigger tank may help his issues. More room to swim, with lots of hiding places helps alleviate boredom, which usually manifests in tail biting.
If it's rot, clean water is the best remedy, so a properly cycled 5g maintained with appropriate water changes will certainly help fin rot.