Thanks for everyone's well wishes.
I booked the appointment for tomorrow morning with the most competent rat Vet at the place. I confirmed they use isoflurane to anesthetize first, then an injection. They will let me sit with him the entire time and they will let me take him home afterwards.
It is very sad, but my old boy is very old and had a ripe life. I'm glad he won't have to suffer.
<3