A 6-month-old infant is brought to the GP with a 3-day history of watery diarrhoea (5-6 times a day) and occasional vomiting. He is breastfeeding well but has had fewer wet nappies than usual today. On examination, he is alert, his mucous membranes are moist, and capillary refill time is <2 seconds. His temperature is 37.2°C. According to NICE NG29 (Diarrhoea and vomiting in children), what is the most appropriate initial management advice?