My step son

So i have a question or advice!!

I am a step mom to a 7 year old boy who lost his mom almost 3 years ago!! I took over and love him with all my heart like hes mine,anyways he is a smart boy but he dont listen in school or to me the only person he (sometimes) listens too is his father who isnt his real father he took custody when my step sons mom passed away (he thinks my boyfriend is his father)! Anyway he throws things,he dont care what he does he thinks hes the boss, ,he tells the teachers what hes gonna do,he throws chairs at school,he still pees his self and we try putting pullups on him at night and he takes them off during the night and we cut drinks off at 8 since his bedtime is 9! He dont wanna do his work in school he wants to color and draw and hes happy!! We don't punish him cause we dont believe in hitting a kid because that teaches him to hit other people!! We took him to the drs and they blame ALL THIS on his mother who passed away saying he misses her which is true but this behavior shouldn't be from that!!:\ we love him but me being his step mom and never had to go through this cause my kids are 3 years old and 8 weeks!! What does this sound to you mommas and do you have any advice