I personally would want to be told. You're basically changing the rules of the relationship without informing him of it which isn't fair to him at all. He has the right to decide what kind of relationship he wants to be in, and if that doesn't match up with what you want/need then he has the right to walk away and find it elsewhere.
In situations such as these, I find withholding information to have the same consequences at out-and-out lying about it. You let him keep believing it's just sex/FWB while actually having a more real relationship going on, and when he finds out he is going to feel betrayed. You tell him, he gets upset at first, but eventually calms down to make a decision about what he finds okay and what he absolutely can't handle and you may have a rough patch but a complete loss of trust isn't likely. Which is easier to deal with? Usually option #2.
You want to balance trust and your husband's feelings? How is he going to FEEL when his TRUST in you is proven false? Dealing with the issue now has got to be easier than dealing with this issue, his feeling of being betrayed, AND the need to rebuild trust later.