I wouldn't mix food with formula and put it in a sippy cup. Even if you make it pretty runny, it's still thicker than plain liquid and could gag him. Formula by itself is much more filling and has more calories than most solids anyway. What's his routine like these days? IMHO, your best bet is to try and up his formula until he decides he's ready to eat solids again. If he'll take more in each bottle, you can go that route, or you can re-introduce a cluster feed in the evening or a dreamfeed and see if that makes a difference. My guess is that it's a phase and he'll work through it in a few days.