I still haven't seen a ruling that says immunities can be lowered, except in a few specific cases that actually say so. Example golem's magic immunity.
I see that SR can be dropped, but nothing about energy resistances, which are a separate abilities.
Then you didn't look very hard:
]Even a character with a special resistance to magic (for example, an elf’s resistance to sleep effects) can suppress this quality.
There is the general rule for your perusal - all resistances, immunities (note that the example given is an immunity) and the like can be suppressed. Unless the quality explicitly says so (specific beating general, of course) this is the case.
edit:
Technically the red dragon cannot supress its fire immunity to be harmed by lava, since it is not a spell effect. Unless it is magic lava I guess.
Possibly true, if a bit nit-picky
But the idea that he could suppress his fire immunity to receive a spell is there, so the implication would be that he can suppress it in other circumstances.