If he's sick or starting to move around or going through some other developmental stage then that could explain the reduced interest. Honestly, the best thing is to just offer and let him feed if he wants to and don't if he doesn't. Or even just wait till he asks for it - they know what they need. They are very efficient feeders by that age too, so 1-2 minutes can be plenty of time for a drink.
If you want him to take more milk during the day, you could try dropping that 10:30 feed and see if he's a bit more interested during the day.
Try not to stress and follow his lead