Poor Elvis won't know what hit him when you get through!

A clean cage and new toys and a loving home (with a squirrel for a sibling,

). He's going to be VERY confused.
I hate to see people treating their animals like that, too. Makes me want to throttle them. He's probably cage aggressive because that's all he had that was his own and his only place of security, even if it was disgusting and filthy.
Jade was VERY cage aggressive when she first came and while she still wants us to understand that's HER cage and HER toys and HER stuff (and we respect that or bleed,

), she's much improved. Don't poke around his stuff any more than you can help. Talk to him while you're changing his paper and filling up his food and water and tell him what you're doing. I tell Jade, "Maid service!" when I work on her cage and while she watches me VERY carefully, she doesn't peck me anymore when I clean or even when I do something like take a perch out to scrub it. She's used to the routine now. Sometimes for fun she'll grab the edge of the newspaper and play tug of war with me when I'm cleaning, but that's just a game.
The way we got to that point was I took a few nasty bites at first. I had bruises on my arms and hands!

But I didn't let it stop me and I just pretended she wasn't hurting me (though inside I was going "YOWCH!!!") and went on cleaning and changing food and talking to her as if having her try to eat me was perfectly normal.

It really didn't take very long, maybe a week or two -- and maybe my brain has mercifully clouded my recollection

-- and she figured out what was what and we've been fine ever since.
Don't try to dig him out of his cage. Just open the door and let him come out when and if he wants to. Trust me, he'll want to eventually. Bribe him with a treat if you have to. But reaching in and trying to force him to come out will just make him worse. The important thing right now is that he feels safe with you and learns to trust you. Letting him have his space in his cage as his own territory will help him do that.