I'm sorry to hear about your mom. Unfortunately the losses do seem to come together.
It sounds like you're doing all you can to find him, and we can only do what we can do. I know exactly what you mean about not being able to properly grieve. My first cat died shortly after my youngest went missing. Its two completely different set of emotions. The death is very raw and everything just kind of comes out at once. But when they're missing it keeps you up at night, never really leaves your thoughts, and its a really empty feeling.