It depends whether you expect him to die on the foreseeable future. If you do, don''t sever as you will automatically own the whole of the property. However, the reverse would also be true. To sever, you just send him a notice of severance. You can find one here:
I''ve recently applied to sever our joint tenancy as I didn''t want him to get my half if I die before the house is sold. My understanding of a joint tenancy is that if one of you dies, their share goes to the remaining joint tenant and not into the inheritance pot for the second wife or whoever is in his Will. I''ve just had my Will re-written too, now that I am divorced.