How exactly? You didn't mention an attack on the Philippines. We're supposing here the US is defensive, not offensive. So unless Japan attacks the Philippines, the only aid I would see would be economic help, naval assets, aerial assets, and military equipment.
Well, given the 'lightness' of OPs post, we don't really know exactly what is happening.
His comment about the Japanese fleet 'obliterating' the RN, I've taken with a pinch of salt (well, a bucket of salt) to assume he means the Japanese fleet attacks British and Dutch holdings in the south Pacific - because of course he knows they can't sail around Cape of Good Hope!
So whilst I agree, the US, in theory, has no reason to declare war on Japan, Italy and Germany I think they will. The briefing, to Roosevelt, will go something like this:
"Mr. President, Japan has attacked Singapore, Malay and Burma against the British and most of the DEI. No formal declarations of war against the British or Dutch governments have been given but its clear that a state of war exists between them. Military intelligence suggests this is a co-ordinated attack by the Axis powers, of which Japan is now clearly one of, to knock the British out of the war. Combined with attacks in the Mediterranian, and German and Italian efforts in Greece, Syria and Iraq, we believe that after Germans failed attempt in September to directly attack the UK, that they are persuing a strategic of indirect attack and strangulation of the UK.
Our current estimates are that Japan will take Burma, Malay and Singapore within three months, and threaten India thereafter. Germany and Italy will likely dispose of Greece in the spring of 1941, before driving to the Suez whilst similtanously mounting attacks from Syria and Iraq. We estimate that by the summer, the UK will have been cleared out of North Africa, the Middle and Far East and be under attack in the horn of Africa, India and Australia. Further, unless something is done, we estimate the UK will have dropped out of the war by the end of 1941 at the latest; on potentially unfavourable terms to us.
Mr. President, we cannot rule out the possibility that large parts of the British fleet could fall into Axis hands, either indirectly by treaty surrender, or directly should the British government be overthrown and replaced by one led by *checks notes*... one Oswald Mosley."
Now a lot of this is probably bollocks, especially as WE, in our twenty twenty hindsight world know the Soviet Union are going to be clobbered, but Roosevelt doesn't want to hear a realistic assessment. He wants to hear 'worse case'. Because he's won the election, he's got a free hand for four years and he wants to DOW.
In this Alt-December 1940, the situation painted above is deliberately bad, but it also isn't one that APPEARS unreasonable. WE know Germany is incompetent. WE know Italy couldn't defeat Greece and WE know Japan will run out of steam and WE know that Spain is a wreck, even if it is on the side of the Axis. But the US really doesn't. A glance at the map shows the Axis, of which it still considers the Soviet Union to be a defacto ally of, in control of EVERYTHING from Coruna in the West, to Vladivostock in the East. From Narvick in the North to Port Moresby in the South. The Axis control half the world at this point and look to sweep up another quarter in the next twelve months (tops).
The United States will DOW. It has to. It only has two choices:
1. Declare war on the Axis; or
2. Potentially allow the Axis control of a fleet larger than the US fleet and domination of everywhere outside of North and South America. And even South America will be uncertain (re: Argentina especially).
The US can't allow 2 and military aid to the UK simply won't be enough at this point.