M.R.
I lost my father when I was very young and I think it really effected my brother the most, although he was much younger than your son.
Keep in mind that your son no longer has his father and especially with a long illness, there is a lot of trauma and major life changes, not to mention his hormones going crazy and he's at a very awkward age being almost a teenager.
Have you spoken to the school counselor? Pediatrician regarding therapy? Thee are grief groups out there, although he may not agree to go. ANy male in his life he can look up to?? Uncle? Grandfather? A best friend's Dad?? It is most likely going to be an on-going process of dealing with his loss. Sounds like reaching out for professional help to get good sound information may be best right now. And....so sorry for your loss. It is something that you never get over, but does get easier over time. My father died nearly 40 yrs ago and when I look back, I can definnitely say that it was THE most life-changing, traumatizing loss in my childhood years. Good luck and I will be praying for you and your family:)