I think he would get a declaration of war. Not the unanimous one he got OTL, but the Philippines are American, and IIRC much of the public felt a sort of... what's the word... big-brotherly sort of bond with the Philippines.
So the US goes to war. And here's where things get interesting.
See, the US Navy's strategic plan was to sit back and wait for... about two years or so, as of December 1941. This would allow them to start gaining warships from the Two-Ocean Navy Act to give them numerical superiority over Japan for good, and just as importantly to finish up a large run of logistics vessels, particularly oilers, that would allow them to push against Japan from the sea. It's not a coincidence that Tarawa happened when it did, nor that Guadalcanal was called "Operation Shoestring", depended on local bases in the South Pacific, particularly Espiritu Santo, and lacked any of the old battleships whatsoever. The problem was, this meant that there was no way the Army was holding the Philippines, no matter how much bluster MacArthur put out. The Navy's worst-case scenario was being demanded by Washington to sortie and try to relieve the Philippines, as that would be playing right into Japan's hands by neatly fitting into the Kantai Kessen plan.
ATL, the Philippines are being invaded, they have a battle fleet, the US Navy is very likely going to be forced to go relieve the Philippines. Now, let's make one thing clear: the Japanese battle plan is... well, it's full of wishful thinking, and outright considers many of its battleships and all of the carriers expendable. The scouting would likely be terrible. Their seven line battleships are, with the exception of Yamato, generally inferior to their American counterparts. On the other hand, the Americans have malfunctioning torpedoes, no way to coordinate carriers, lousy night-fighting capability, and they're off their own playbook.
What I'm saying is that it's going to be a fucking bloodbath, and either way, I seriously doubt the US Navy is going to be able to prevent the Japanese from overrunning the Philippines and a Southeast Asia. However, win or lose Japan is going to be largely out of ships, and at that point it all depends on whether public opinion can sustain the war while the US rebuilds the Pacific fleet. If it can, then the US steamrolls Japan in 1943/1944 and is knocking on the Home Islands by 1945. If it doesn't, well, Japan may actually get a negotiated peace. I'm leaning towards the former, myself.