Despite his mental challenges, Brian is clearly acting in congruence with his feelings by going into his grandfather's room. Brian is bereaved and needs comfort. This raises the question of how the rest of the family is handling their grief. It wouldn't hurt for Brian to see and hear some of that in a form he can comprehend. 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.