I have a 3 month old boy who I have breastfed u...

I have a 3 month old boy who I have breastfed up until now. In the last few weeks breastfeeding has become a real challenge and as a result my supply has gone down dramatically and my baby has failed to put on any weight. The clinic told me to complement him after each feed and for the first 2 days I was able to express enough to do this. Last night I had to top him up with formula for the first time and he guzzled it down without any fuss. Then he fussed about at the 4am b/feed (which is usually the calmest). I am devastated as I just assumed I would breastfeed for at least 6 months. I think he is weaning himself off me. I only want what is best for him, but am so upset at the thought of not breastfeeding him. I was wondering if anyone has had any similar experiences and what the outcome was. Thanks