For now take away that feed and just give him sugar water because if not he might get worse...not sure though..because it could very well be something else. I am just going by what I just went through this weekend. I have heard that game bird feed works just as well for geese...but I know that mine started stumbling and stuff and sugar water would snap him out of it but then it would happen again, plus he wasn't eating or drinking...well first he was drinking tons ...then after another fit he stopped drinking all together. I had to drip drops into his mouth and he snapped out of it . The next day I gave him the new feed (no feed the prior day..only greens and sugar water and he didn't even eat the greens) and the new feed started making him feel better...by 24 hours he was a different gosling...but again I called the vet and they told me based on the symptoms they thought it was coccidiosis(even though there was no blood in the stool) so I treated for that the same day too. It's hard to say whether it was the medicine or the food(even though I haven't given him but one dose yesterday and he's due one tonight, he has been acting fine...so that tells me it could have been the deficiency). I needed the assurance of the coccidiosis because I had two goslings arrive today and I didn't want the risk of there being coccidiosis and it being contagious to the new arrivals...so just in case i treated it.
does he have diarrhea? what does it look like?