Aww! Froggy!!! He's so cute! It's really hard to get a good picture of these guys, and there is quite a bit to look for health-wise that simply wouldn't come up on a picture. Even between flash and flashlight - I can't judge on coloration, but he does seem a *bit* on the thin side to me in one of the pictures - again I could be wrong, it's really hard to tell from these images. Is he eating well now that he's on his own? I believe I sent you a PM with information on what to watch for in a sick ADF, but from here *shrug* He looks okay to me! ;)
A misconception here - your frog isn't lonely! Regardless of some beliefs to the contrary, ADF do not NEED to be kept in pairs or groups. They are social, and will behave differently with friends in the tank, but provided that they're well cared for, they are perfectly fine and happy to live a long life as singles. This is not a creature that requires
the company of others for it's health.
That being said. . . I wouldn't be comfortable with putting 2 frogs in anything less than a 5 gallon tank - with filtration and heat. They need the room to swim around, and with the messy types of food that they need to be healthy it can be very difficult to keep a smaller tank stable enough in parameters for their overall health.
Particularly to you, I'd wait a bit longer and keep an eye on the one you have. Get used to his behavior and plump him out a bit before adding a buddy. . . you can save up for the 5g while you wait! Chytrid is a very real problem in the world just now, so personally, I have decided to wait a period of 3-4 months to be sure that mine wasn't infected with it. Now I'll be QTing a 2nd frog in another tank for the same amount of time before putting the two together - I'd be devastated if a new frog came in sick and harmed my first - just something else to think about here. . . if I were you, I'd wait until I could get a 5g up and cycled, then QT #2 alone in THAT tank for a period of 3months before putting them both together. It sucks, but patience is ALWAYS worth the wait in this hobby, I'm learning!
On 2.5 alone vs 3g with Jaws - I'd leave Mr. Frog in the 2.5 by himself for the moment. As you've learned, it can be difficult to make sure that an ADF eats well in community with other creatures, and most people find that they really do best on their own in a species tank. Also. . . as you've seen already, ADF and Betta fish aren't *always* the best tank mates. A lot of this depends on the temperaments of both frog and fish. Then there's the added stress of another move to consider. . . I'd just leave them where they are for now! As long as you're closely watching/testing/cleaning the tank that he's in, 2.5 should be fine for one little frog. Plus, you'll be able to get a better idea of what is normal behavior for your frog and get used to his behavior and habits - which might be very different if he's in the company of a fish!
I'm sure someone with a bit more experience will be by soon with some more seasoned advice, but that's my two cents - hope it helps!