A 14-day-old male infant born to a 32-year-old multiparous mother is brought to the clinic for poor weight gain. Birth weight was 3.2 kg; current weight is 3.0 kg. The mother reports breastfeeding 8–10 times daily for 10–15 minutes per side, but the infant appears hungry after each feed and cries excessively. On examination, the infant has dry mucous membranes, sunken fontanelle, and poor skin turgor. The mother's breasts are soft with no engorgement. Lactation assessment reveals shallow latch with clicking sounds during suckling. What is the most likely diagnosis and the best next step?