If he's eating regularly that it's fine, always ask the health visitor but it will be probably fine. My son didn't drink milk at all at that age just a bit in the afternoon when I was breastfeeding but never took the bottle and by 16 months I stopped breastfeeding and he totally stopped getting milk because he didn't want the bottle. But because he was eating solids was totally fine. I'm sure your little one will be good but just ask the HV to be safe :)
Try to give him high water content fruits and vegetables that will.help or offer him milk in not a bottle whatever cup he likes he might just wanna do it himself now 😊
Is he eating more solid food perhaps? Teething might also be a factor - my daughter would sometimes go off her milk or food when teething. As long as he seems okay within himself and doesn't show signs of dehydration then it's probably just a phase.