Poor DH and you.I think its still early days.It is very hard to watch a loved one deteriorate.Cancer is truly a terrible disease.Your dh is probably still not able to talk about his fathers death and evidently using alcohol as a crutch.I have seen a lot of varied reactions to death (I work on a cancer ward).Everyone reacts differently.Could he take some compassionate leave from work?
It might be that he is getting things together in his own mind.He will think of his dad and possibly be mulling over their relationship.It is so hard to watch your parent,who has usually been a focal point in your life become weak and vulnerable.There may have been things he wishes he had said to his father that he didnt.
Try to give him opportunities to talk about how he is feeling.It may take a while.Possibly his dads funeral will give him a chance to accept the reality of his fathers death.
If you feel that he is getting to a point that he is truly not coping in time I would suggest an organisation called CRUSE.If your FIL had a Mcmillan nurse they can also do bereavement counselling if you feel your DH would accept this.Or if you contact your local hospital they will be able to refer you to a McMillan nurse.
I would give him time and love to start coming to terms with his dads death,but if it starts impacting upon your relationship or that of your children it is time to call in additional help.Hugs.