Currently, you are not related in law, so if you give him half a house, even if the house is mortgaged, it is deemed a gift and he would be liable for Capital Acquisition Tax, of 20% of half the value of the house. This would not really be worth the money, even for peace of mind.
If you die and leave him the house in your will, he will be liable to tax also, unless he has been living in the house for 5 years and owns no other house.
If you are worried about your financial future, in the event of his death, you need to think about life insurance.