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Your Son would not automatically be entitled to join in as a co-Executor, just because he had a Lasting Power of Attorney.
What you would therefore have to do is to prepare a codicil to each of your Wills, appointing him as an additional Executor on the first spouse's death. A Solicitor is likely to charge approximately £200 plus to prepare both codicils.
There is really no practical difficulties in having 2 Executors- it just means both the survivor and your Son would have to sign all the necessary paperwork, as opposed to merely the surviving spouse having to sign as would be the case as things stand now.
If you don't appoint your Son as a co-Executor, he could of course still assist the surviving spouse with the paperwork, but would not have any authority to speak to the Banks and any other institutions, as they would only discuss matters with the appointed Executor.
I hope this assists and clarifies the position to you.