He will not be able to live there rent free if he survives you. He has no automatic right to do so. On your death your house passes to your children, if you've made no will, or as to whoever you've given it to in your will. The new owner is entitled to vacant possession and can evict him.
You should make a will giving him the right to remain there for life, rent free (or such other period and conditions as you require) , after your death, though granting ownership of the house to your children. You should also know that , if you married him,that will would become invalid and of no effect and a new one should be made after the marriage ( or expressed to be of effect only when the marriage takes place) . In short, you need legal advice from a solicitor to ensure that your wishes are carried out and the necessary document(s) written.