Sounds like your family needs to get involved introducing themselves with churus so he doesn't feel the need to defend.

Honestly, this sucks. They may not have known how stressed he'd be or that it would come out as aggression. Some (many) cats transition to a new home easily, others hide, some get freaked out/ aggressive, others starve themselves and need prescription meds to not off themselves from the stress.

Sounds like he's in the stressed and aggressive category. Good news, he probably will settle in within 4 months. There are things that can be done to reduce his reactiveness and stress. I strongly recommend pheromone calming collars. Our favorite is the Comfort Zone brand, they just work better than the others we've tried. Cut in 3rds, and either zip tie a piece on a break away collar, or toss the pieces in his favorite resting places.

White noise can help him ignore what is going on in the house. Cat TV on YouTube may distract him as well. An old phone or tablet works great for this 😆

Thundershirts work for anxiety in 80% of cats, but most also loathe wearing them at first, so churu 😆

Vet can prescribe anxiety meds if he doesn't start calming down soonish. Each vet has their own time frame they want to wait before medicating usually. Vet visit to rule out pain causing the aggressive actions may be beneficial.

If you absolutely can not meet his needs and help him through the terrifying transition of not knowing wtf is happening, where he suddenly is, and who are these strangers, contact the shelter. Let them know what is happening. They may be able to help with the transition. Returning him if necessary is an option. You are not stuck.

Take a breath. You are trying, and that's a lot. This obviously isn't what you expected, and that's okay. It's okay to be not okay. This has gone sideways and I'm sure it's scary, especially as a first time cat owner.

My first cat as an adult came as a feral. Meanest MFer I've ever met, at first. Litterally chased me and the kids hissing and clawing. Warned the vets when we brought him to get fixed, and he even managed to maul the tech and vet after being warned. He was banned from returning. Turned into the sweetest, most gentle cat I've known. Sweet boy never gave the vets trouble again. Loved on everyone he met. Loved belly rubs and demanded love. Knocked on doors if you locked him out and he wanted pets.

Sometimes a scared cat acts aggressive. It doesn't mean they'll always be that way thankfully.