I kept my ds off his formula at around 11 months for the same reason but he only had 2 feeds a day. He still ate solids but his drinks were water, well diluted juice instead of formula. It was only for a day or so and then he started his formula again. It was just a suggestion though - you dont have to do it if you dont want to.
DD has viral gastroenteritis at 10 months (dirrhoea, no temp). Lasted about 5 days and we were advised no dairy. Once we cut it out she was better quite quickly. Fortunately she was bfed so reverted to a liquid diet for a while.
The formula will keep him hydrated and that's the main thing. The cutting out dairy I would think is a good measure for a baby who isn't getting over the virus and is heading for dehydration, which dd was (she wouldn't take water at all). Turned out that a liquid diet of just bmilk for a week was fine and she actually gained weight.
She's had it twice in her life, 10 months and 12 months and it took around a week for her to get better.