Mourning is appropriate for people of all ages, While we all grieve at a different pace, children included, most of us eventually go on with our lives. Children receive permission to grieve from adults. Perhaps you could sit with Brian and together remember your father by looking at pictures. Be sure to allow him to release his emotions as much as he wants to. It will pass.
Pictures can often be a key to communicating for a nonverbal child.
Perhaps you could find a children's book about grief that relies heavily on pictures. Your librarian or funeral director or a local member of the clergy or Brian's school counselor might be able to help with this. Do take time for yourself to allow your own feelings to surface. Do express them to the people who love and care about you. It's also a good idea to let Brian's teachers know about your family's loss.