4 year old boy temper tantrums

My 4 year old grandson is a wonderful young man.  Very articulate and intelligent.  He has started headstart and is doing very well.  Very well mannered and behaves well at school.  His mother is pregnant and is expecting twins.  In addition, his mother is in a relationship with the father of the twins and he has moved in with my daughter and grandson.  Lately my grandson is having temper tantrums more particularly at bedtime.  We know that there are a number of adjustments he is making, but his tantrums are most commonly directed solely at his mother.  She is physically limited in disciplining my grandson due to the pregnancy (which has been high risk from the beginning).  Does anyone have any ideas on anger management / behavior modification techniques for my grandson or parenting techniques for my daughter?