I would think of it in terms of personal integrity and an obligation to self-care then, if you want to think in ethical terms. By participating in something that someone else will perceive as harm to them (even if you do not) then you are morally implicated in that harm.
Consider the case of someone who forcibly converts another person to their religion. The converter thinks that this is benefiting the person by converting them to the one true religion and saving their soul. The person being coerced perceives themselves to be harmed by the coercion and the forced suppression of their own chosen beliefs and culture. You can make an ethical case that it is the perception of the person being harmed that is most important. A child restricted in their behaviour for their own safety might disagree, but their reduced capacity to make decisions is a factor there. The girlfriend of your boyfriend might make different (and better for her) choices in regards to her boyfriend if she knew what the real deal was, and your boyfriend is depriving her of that opportunity.
In terms of obligation to self care, I agree with the others that people who lie to their lovers lie to their lovers and this person will likely be dishonest to you in the future. You may wish to think of this situation as a test of his integrity, so you have an accurate picture of how he behaves.