Well when I told my ex it just felt right at the time, it was like a build up and it just sort of happened.
In your case I would suggest telling him when you are both alone and comfortable as it seems he already has an idea something is up, being open about it now would also tell you what kind of man he is, do you really want to wait months, years down the line only to find out then he has a problem with it?
Better to do it now than later on I say.
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-SF