My husband makes fun of my child's weight. What am I supposed to do?

You should defenitely talk with him. Choose a nice, quiet moment where both of you have time. Tell him that you are concerned and dont want him to make fun of your child. If he wants to brush it off as a joke tell him that these comments might still hurt your child, especially as he is a role model for him. Tell him it is his job to be loving as a (step) parent and not making fun of your child. This might seriously hurt him and might stick with him for a long time. So tell you husband about this and make sure he gets the severity. If it is not working you should consider a divorce or break if it is really severe. Btw dont forget to tell your child that you are there for him/her and that you support him/her no matter what and that you dont share your husbands opinion.