My 16 yr. old cat is acting totally different since his 2 brothers died-18, 9 yrs. old-he is totally dependent

Recently my 18, 16 and 9 yr old cats were exposed to a virus at the vets office when they went in for yearly boosters. In a matter of a week my 18 & 9 yr old died. The emergency animal hospital was able to save my 16 yr old but he has now gone from a totally aloof, almost invisible cat (basically the wuz of the group and controlled by his brothers) to one who does not let me out of his sight. I know he was scared because he had never been sick before and never away from home overnight - now he won't go to bed unless I lay down with him and he puts his head on my lap, he will not jump up on the bed - he meows until I pick him up, and this weekend my granddaughter was visiting and he sat beside the bed and meowed all night because she was sleeping in the bed with me. My husband tries to hold him, but he is basically indifferent to him now - before they were great buddies. I AM LOST AS TO WHAT TO DO!!!!