Fox_13 made a good point. So thumbs up and take note of that.
Next , I would like to add that dreams are often just the ramblings and wandering of the mind. Subconsious Fears , Desires , Things that are often in your mind tends to manifest in dreams.
Thus , in my opinion , a spell is not required. Have him banish such thoughts away from his mind , not think it at all and it is likely not to appear. Simply have an application of will and desire and a nice sleep of oblivion is easily achievable.
I would not suggest Lucid Dreaming for this. That is like way to overkill. I personally would have your boyfriend keep a dream journal. When he wakes up , he is to write out the dream. Do this for a week or two , take note of recurring themes , tie it back to his own subconsious fears or other thoughts he haves and cease them.
Also , I know you don't want to hear this and I hate to be the one to deliever bad news but have you ever thought that it is a nicer way that your boyfriend is attempting to break up with you? Perhaps his rationale is if he keeps hinting on things such as "I dreamt we are drifting away" or "I dream that our continued future together is not healthy for you and me" and etcetera is that you would take the hint and end the relationship nicely? I mean you said that he keeps bringing it up , so this may be a possibility. From what I infer anyway.
There are plenty of ways guys try to end things nicely with when together with a girl. Phrases like "It's not you , It's me" or "The feeling is right but the time is not" or "I had a bad dream about us not having chemistry anymore , do you think it is a sign?" are not too uncommon.
Don't get too upset when I suggest this or chew my head off , but keep in mind that this is a possibility.