Agree with Jacq. Additionally, try to surround him with things that are comfortable or familiar (music he's been hearing since he was a newborn, a favourite blanket or toy), and try to talk to him and tell him what's going on.
Try to have him eat lots also of regular food so he won't cry out of hunger. You can also distract him with trips to the park or by playing with a pet.
Lastly, stay calm and hang in there! I will wean in a month's time and both my sister and sister-in-law said that the non-stop crying lasts for only 2 days then the child will get the message that mummy has no more milk or that he can't drink from mummy anymore.