He may just have a bad case of gastroentertis, but his age would make me wonder if his symptoms are due to an age related disease, simple kidney and liver functions will better assess your
dogs overall condition. If it is
gastroenteritis, then typically antibiotics should help your dog feel better, in about 48 hours, give or take a day.
You said he took a biscuit, but turned his nose up at the prescribed dog food. Try using his normal dog food, and see if he will eat it.