It's best to start off by asking indirect questions such as, "Why do you suppose you..." and then fill in the blank with the bad habit. Your question might take him off guard. It could be that he didn't even realize he was doing it.

Or he might not have known that what he was doing was a concern to you. By asking a question, you allow him the opportunity to think about a solution, instead of immediately feeling like he has to defend himself.

However you should also be prepared for denial. Some people just really have no clue they are doing something that has become a bad habit. If he denies it, be ready to give him specific examples.

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If you generalize his bad habits, it will be harder for him to see them as real issues. However, if you have concrete examples of behaviors you think are toxic, then he has a better chance to understand and make concrete changes that you can track.

When you share your examples, don't use it as an opportunity to vent. Talk in a calm and gentle manner. He will be more receptive to what you have to say.

At the same time, you want to express why his bad habit could be destructive to your relationship. If you are going to bring up what could be a sensitive topic, there should be a valid reason for it.