We'll get racks to have more trays planted. We offered one tray of grown grasses today, but it had been eaten out quickly (probably by the other two geese in the flock). We have enough seeds, just not enough space with warm temperatures. The racks will improve the situation.
I won't mix the food into the grass seeds for now (based on the past experience, it's more likely he'll stop eating grass seeds, too, than he'll try the other food along). I think it's more important that he gets nutrition than I try to get him back to a normal diet. I'll wait for the vet's diagnosis tomorrow to see what caused his conditions before making sudden change.
It’s important to remember that geese are designed to eat grass in quantity, so a change in diet needs to reflect this. Mine will often refuse something new several times before they accept it as safe food, so options are important when offering something new- geese are very cautious about new things including food. As a good place to start given your limited facilities for sprouting, start some grain germinating- in a bucket is ok and offer some once the starches have started malting.
Initially it has a slightly sweet smell as the enzymes get working but changes to a slightly sour smell- like sourdough bread as germination progresses- turn it twice a day and rinse the grains, drain well.
If you haven’t had veterinary advice regarding antibiotics here then please seek it, misapplied they can cause a lot of issues including digestive upsets. Digestive processes are almost entirely dependent on bacteria, so a broad spectrum antibiotic can significantly reduce the digestive ability of the animal