I use the baby prune juice (no particular brand), but I think it's actually prune-apple. I wouldn't put it in his formula bottle, though, because I don't think it would taste good. Try giving the prune/water mixture in a separate small bottle. Some babies take to it better than others. My baby is nearly 20 lbs at 5 months, so obviously he's never seen a food he doesn't like, so he pretty much drinks it down just fine! However, I do give it to him with a higher flow nipple so it goes down quicker. Try 1 oz at first and see how LO does.