Find out WHY he feels this way, something is upsetting him,does he feel left out of the family group, many step children do feel like this in a mixed family, focus on his positive side ,give him some extra one to one time so he feels wanted .If he seems disturbed ask for some counselling help.How does he do at school hads he got friends, get on with teachers., something is triggering it, when you say you dicipline him for his attitude ,what would the attitude be,and what would the dicipline be, Teens do have attitude its very common ...
Take him a psychologist asap before he acts on those drawings.