The not eating is a big problem for intaking vitamins. Force feeding is a great idea, but can be difficult with the Pyxies (especially if they have any size, be careful or you'll find out about the teeth you can't see on the top jaw. I would suggest Sticky tongue farms 'Vit-all' as a great supplement. Repashy makes a calcium +D3 that also has vitamins I believe...not sure though.

Why do you say he'll need vitamins soon? I'd say leave him be. Don't stress him just when he's doing better. Force feeding is pretty stressful....just sayin.

What is he eating? You said he's eating something. What is it? What was the chitin from? If he's eating, just put the vitamin on the food.