How can I restore access to my son's blocked iPhone?

He will need to take the phone and the phone's receipt from a genuine Apple store or authorized reseller to an Apple store to have them reset the phone.

Smagulov wrote:

I have a receipt for the purchase and can provide it. His grandparents gave it to him as a gift and now he is worried and does not tell them or his father about it because he is afraid. Please help me with something.

As long as the receipt is from Apple or an Actual Apple Authorized Seller, they will help with valid proof. If the receipt is not from Apple or an Apple Authorized Seller, they won't help and the phone will be useless. Your son should be on the lookout for a ransom as that is very common. They lock you out after you enter their Account, then they tell you the only way they'll release the account is if you pay them to unlock the account.

Is the receipt from Apple or an Apple Authorized Seller?

You will need the original receipt that shows your name along with the identity of the phone and you can get help from Apple to remove the activation lock. If you can’t get that done, then the phone is useless to you. Recycle it and get a new phone.

If you do not have the original purchase receipt, the phone is useless and was essentially "gifted" to someone else when their Apple Account was entered to allow locking the device to them. I valuable lesson to learn. The internet is full of scams and trusting strangers in game chats or even Social Media can have catastrophic consequences. Thankfully in this case it was just a phone that had been gifted to them and it could have been much worse.

This is a user forum. You are not talking to Apple. You need to contact Apple support directly.