How To Avoid Dehydration In A Baby?

Lat night, my 10 months old baby started vomiting and then we take him to hospital and doc gave him digene and lanzol 15 But today he is not having anything except milk I wanted to know what should i include in his diet to avoid dehydration and what should be his diet plan? Also wanted to know that is it safe to give formula milk to baby while sleeping s as to make him sleep?