Clean water and aquarium salt would work. I had a neighbor to come in and take care of mine. She fed them 2 pellets apiece every other day. I didn't really worry about too much or too little food because she has fish and knows not to overfeed but I thought about mine every day and wondered how they were. I missed seeing them every day. It took me 2 or 3 days to recover from my trip. lol
Yeah, I would just go ahead and treat him in the 2 gallon. When I used to have just the 2 original fish I started out with, whenever I left to go to my brother's for Christmas, I would leave 1 fish at a neighbor's, who had a betta and friend of mine took the other one to Indiana with her for Christmas. The neighbor wouldn't keep both of them because one used to belong to another neighbor that she didn't like!