He refuses to give back laptop, what to do?

At the beginning of 2017 I ordered a laptop via Amazon. I had this delivered directly to my nephew's home because he needed him at the time. Accordingly, my address is in the billing address and his address in the delivery address on the bill. It was intended to buy the laptop for me. But it has never come to that, there's no contract. Occasionally he gave me a few euro, but the laptop cost me over 800 euro. I bought the laptop especially for him because he did not have that much money at that time.

He has not paid the laptop until today, but he does not give it back to me. Currently he has blocked me even everywhere.

I realize that I can't ask for money without a contract, but can I get the laptop back with a lawyer? Actually, I would have to be the owner and he just the owner or are there any problems because the delivery address is on the invoice?

Can I do anything to get the laptop back or just give it a lesson? I do not care about the money, all I want is that he gets consequences for his actions.

No, there are no problems, you are clearly the owner.

I would just bag it on my next birthday coffee. Should he prove that it did not go right?

He would get him again for a ransom.

We have no contact anymore. He even canceled the Christmas dinner because of me.

Then tell him about theft. Or at least threaten it.

You write that you do not care about the money. Then do not worry about it… Leave him the laptop and it's good… But: At some point, this nephew will need again (financial) support or help, which you can then decline with a clear conscience and with absolutely understandable justification

The most instructive consequences are those that are not yet announced but for the "common sense" so completely understandable. Let him feel "strengthened"… He will change his actions and his way anyway, if he "falls on his nose". He does that the fastest and most reliable, if he does not suspect that the required "support" is missing.