Hi,

Was it your desire to wean now, anyway? Was this something your doctor discussed with you?

I'd be interested to know what medication you are on (you can PM me) If the meds you have are like the combined pill - the reason you can't breastfeed is that they will dry up your milk - not that it will be harmful to your 18mnth old. so, if that's the case you could keep feeding for the time being and as the milk diminishes he might find that he decides by himself that it's not worth continuing.

If you had wanted to keep breastfeeding - go back to your doctor and get a plan that doesn't preclude breastfeeding.
If you wanted to wean anyway, you have probably broken the back of it. He's vented his anger, and will probably be a bit better. Enlist other family members to help out with him/distract him. Keep busy for a couple of days. Start a new bedtime routine - read him a book before he goes to sleep. Be generous with the cuddles, and keep explaining that there's no milk anymore
Let us know how you go.