Talk with him
Talk with him about the baby, but make special time for your toddler. Watch what you say- don't say "i can't play with you because i need to feed the baby".
Say "i'll be happy to play with you in 10 minutes". Have some special books or toys your toddler can play with while you are nursing the baby- he can only use them at this time, so he will look forward to that time instead of resenting it.