Thanks.
To answer the question as to why my SIN is on the account, the account is joint and the T5 always shows the first-named SIN associated with joint holders. CRA instructions require "proportionate" reporting and for Line 12100 says " Generally, you report your share of interest from a joint investment based on how much you contributed to it."
In most years, the contributions and interest are roughly equal between my son and I, but this year, this particular GIC was purchased solely with his money. Due to his disability and treatment at the moment the GIC was established, this was the easiest way to create the term.