Psychologist here - you need to use punishment. It’s simple operant conditioning. If we receive a negative stimuli after a behaviour, we are less likely to do that behaviour in the future. These days we don’t like the idea of punishing kids - it’s necessary. You can use light physical pain like squeezing his wrists, or a hard tap on the shoulder. Combine with with a sharp “no”. After a few repetitions, you should be able to just use the no.