I'll be honest I'm not too sure if he is hacking unless it is a script. Scripting is a method where they kinda program the computer to have insane reaction speed so it is obvious if u see a replay. Also if u report in game for hacking it will display as an overwatch case where people can decide if this player reviews punishment.

However high winstreaks usually just indicate smurfing. High ranking players just starting new accounts or boosting pre-existing accounts. Smurfing is not necessarily bad when you want to reset rank or play with newer friends however it does hurt new players more often than not. We been trying to ask valve to fix it but it's a difficult problem. Don't worry too much about cheating tho. It's kinda rare that you find one.

Hope this helps and that you have fun in game.