I have also been putting vitamins in his water since Christmas.
Please be careful with Vitamins. They can actually do a lot of harm, more so than good. Especially if the vitamins contain ANY of the "fat soluble" vitamins: A, D, E, K, these are very harmful in vitamin form, and should only be fed as prescribed by your avian vet.
Toxicity of Vitamins
I would say a very little bit of vitamins can be added to water 1-2xs a week and for example, only in the morning time for only an hour or two.
If Stitch is fed any pellets, that should help give him ANY of the vitamins he needs.
HOWEVER you want to give him some immunity boost, to prevent him form getting sick, try giving him PROBIOTICS, in any warm foods you give him, like AVIBIOS:
Buy Avitech AviBios Probiotic (Lactobacillus) Supplement 4 oz - Bird Vitamins and Supplements Online at Arcata Pet Supplies
You can also make sure he is getting some citrus fruits daily like: oranges, limes, lemon, tangerine, etc.
Giving NATURAL High in Vit. C will do not harm to him. I give ALL of my big guys a orange piece a day. So 1/4 of an orange a day. They typically dont eat it, but drink it