Soon to be stepmom(engaged) needs advice on spoiled 8yr old stepson "joe"?

joe is my bf's son from his 1st marrage.the problem is joe wants consent one on one attention from my bf when he comes to our house,he even has to sleep with joe.joe is fine if he has a freind over to play with but if its just us he makes his dad feel guilty for spending any time with me.his dad can spend 9 hrs straight playing with him(which he does all the time) and then want to sit and talk to me for 15 mins and his son has a fit.he says things like "its my day to be with you i don't like this" i understand this is their "special time" but joe gets to be with dad every other day and weekend and 90% of the time he has his dads full attention. i also join in on games too.i believe the problem in part maybe my bf's exwife.she is very bitter and resentful of the divorce(even after 7yrs,he )and talks badly infront of joe about us.also she consently throws in my bfs face how he abandoned his family.my bf being a good dad feels guilty and gives into joes every wish thus spoiling joe badly.