I really have no idea how much the credits are, but imagine that while important to you are not actually that much. Given that your OH will now be sharing bills, ie household bills with you, which should help the situation, you should notify the revenue. Its unlikely that they will identify that he is living at your address, but if they did surely the results would be far more serious that losing it in the first place. If you dont know the answers to the questions, get someone else to ring the revenue and ask the questions. if my BF moved in would i loose the allowance etc.
You maybe able to get your OH to pay rent under the rent a room relief scheme,someone else may be able to offer further info on this, this would not be taxible on yourself, however the income would have to be notified to the revenue but would eliminate the any gift tax issue, and he would be able to get tax credits on the some of the amounts paid.