A decrease in milk supply is a normal reaction to the hormone levels in your body that are supporting your pregnancy, and efforts you make to increase your supply during this time will probably not be successful. During the third trimester your milk transitions back to colostrum, in preparation for your new baby. Colostrum continues to be made until after the new baby's birth, and it is not harmful to have your child breastfeed at this time — there will still be plenty when the new baby arrives.
While working to maintain or increase your milk supply during this time is rarely successful, your child can still continue to nurse. In fact, he will still get substantial immune and nutritional benefits from the milk that is there. Knowing that your supply will be naturally lower, you may need to add in some formula or additional solids to meet your toddler's nutritional needs during this time. Then, once the new baby is born, your milk supply will increase, and you will be able to provide more milk for both children.