Could be your grandson is having some jealousy issues over the youngest child and possibly feels left out,its tough when you have at that age to deal with,all this stuff,and a Baby, he only sees Dad every other weekend and theres a new boyfriend,it all adds up to a child who needs a lot of positive attention, and one to one time from his parents,He ia acting out because he is unhappy. How about the boyfriend gets him involved in sports, soccer ,Games, and Mom gives him one to one time Bio Dad could focus on his sons positive side..
It is great that you are observing his behavior so well. I don`t have any professional advice or a wealth of experience but judging from my own son`s behavior, I bet your grandson is 1. a perfectionist by nature 2. stressed out about something, maybe the new-family situation.
My son still gets meltdowns now and then at age 6. It always escalates when he thinks he doesn`t master something as perfectly as he imagined and it only happens when he is either very tired or getting sick. I had been looking for emotional uproar initially but then realized, a sick perfectionist is just his own worst enemy sometimes. Maybe it is just that....or the new baby, the missing dad...the boyfriend...stressing him out.