How much solids is he having?
The problem with topping up with formula is that some babies then prefer using a bottle. Drinking from a bottle is much easier then from the breast. Formula is also more filling so it can mean they drink less often. When you then introduce solids - especially quite young, they then get full on solids instead of milk. Their tummys are tiny, so you need to be very picky about what you put into it. Solids are only for experimenting with up until around 1 yr. A baby does so much growing, not just their body but their brain and organs that they need a heap of goodies that are found in bm only. Worst case scenario would be formula. There is no solid in the world that can provide them with the nutrients, fats, minerals etc then milk can. One of the reasons they say to hold off on starting solids is because they can fill up on it. At that age especially you want to get as much bm into them as you can.
As much as he loves solids, it isn't nutritionally sound or a whole food for him at that age. It sounds as though its actually compromising his milk intake which completely defeats the purpose of solids.
So my guess is its a conflict of foods! Their gut is extremely sensitive. The combination of milks could be stressing it, then the addition of foods may also be stressing it - so he might actually not be feeling red hot. That plus the formula and solids filling him up so he doesn't want the milk.
Not sure how much solids you are giving though, so it might be something else. smile