Tracey, the fact that his poos are yellow now is very important. I would suggest you follow Barb's advice, stick to the breast, cut dairy from your diet and I think in about 2 weeks you'll see a big difference.
I had this with my first baby, and by about 7-8 months I could have some dairy in my diet (butter on bread, milk in tea a couple of times a day) and by about 15 months he was over it. We were living in the country at the time, but several years later I met a paediatric gastroenterologist. I told him about what had happened, that I wasn't well supported by the paed, the strategies I used, etc. He told me I had done just the right thing.
If the problem is more complex then they recommend a visit with a gastroenterologist or allergist. If he responds dramatically to something simple like cutting out milk, then just go with that for a few months and reevaluate later.
Must go, crying baby. I'll pop in again later.