Hi, there,
Manta makes some good points. Breastfeeding isn't *all or nothing*, Thee's much more to breastfeeding than the quantity of milk you make.
First up, I would head back to the doctor for a review. If your thyroid is under-active, you are not making enoough thyroxin- you can supplement the amount of thyroxin your body is making with with tablets - and this is compatible with breastfeeding. (a call to Rodney Whyte at the Monash Medical Centre drug information line will be very helpful - som-one will post the phone number I hope!)
There are a number of hormones needed for a full milk supply. Like a cake, you need all of them - but different quantities of each. A good explanation of this can be found at Low Milk Supply -- Providing Information and Support to Breastfeeding Mothers. It is likely that the motilium is not effective,because it boosts prolactin levels - but your prolactin levels are probably fine - it's your thyroxin that's the problem.
As your baby is able to take more solids, gradually you will be able to replace the formula in his diet. There is no need to stop breastfeeding. Hanging out at ABA meetings is great - so often we encounter people that just don't kno why we do it - why we don't *just put them on the bottle* It;s good to feel understood and supported. You are doing a great job - it's tough when you have fed so long and so well, and then face these sort of problems.