Before you tee off review the rules you are going to play under for the day. Beginning golfers often play mulligans, preferred lies etc. With the rules established If he reports a lower than actual score on the just completed hole, walk him through his strokes to help him see his true score. Do this a few times and if he continues to cheat I would stop keeping score or stop playing golf with him.
Another strategy: play dump ask as one or your playing partners to explain a "gimmie put" and how they work.
Watch rules videos so you know what the penalty is for say a ball in a water hazard and then explain the rule in conversation during the round. "Did you know" sort of thing.
If you do or say nothing he will only continue to cheat and your enjoyment of the game will continue to suffer.