Depending on how long your caring for the little man i wouldn't worry too much.
DD1 would have a little bit of milk on her cereal and that was it, she wasn't a big milk drinker unless we added banana etc but she was happy eating cheese and yoghurt.

Aslong as he's eating and drinking other things i wouldn't worry too much unless it's a long term thing and then i would try and find another way to supplement the dairy.
If you can try and stick with water, juice can be quiet high in sugar so is ok as an every now and then thing but not an all day everyday drink IYKWIM.

GL, hope you find something that works... maybe a nice cheese sandwich for lunch or some yoghurt would work just as well.