Firstly at age 67 I expect you have some considerable time to sought this matter out, unless there is something you can't reveal.
Second if you die before your son then it will become a matter for your estate to resolve, how you frame your will, which you should have already prepared, will determine the best way forward, Making a will is not a tedious matter and can be relatively inexpensive if matters are straight forward. Given that you are particularly worried about this matter it would be best for you to see a specialist solicitor who deals in the area of estate planning.
One thing you may have to do is have your son register an interest against the Mortgage. But don't leave it, see a solicitor soon as this will give you the greatest peace of mind, there are also aged organisations who can offer plenty of advice.
You haven't mentioned which state you are in but in VIC, to the best of my knowledge it is quite easy with 3 out of 4 of you problems. Anything that has both names on it will automatically be taken over by the other. You may need to present Probate Certificates to the bank but this is an easy thing to do.
As for the title that is in your name only a couple of hundred dollars can fix that, have the title transferred into both names. Go see a conveyancer and they should be able to help.