An ideal time for switching to twice-a-day feeding is usually around 3-4 months of age, but because your dog is a large breed, you might want to wait until 4 (unless it's already passed that time, I'm too lazy/incapable of converting the weeks, hahaha). It can also be based on personal preference and the type of food he is on as well. If he looks like he still gobbles down his three meals a day, he may not be ready just yet.
If you do switch him to two meals a day and he's scarfing down his food, it's probably better that you wait a little longer to prevent possible bloating.
I could be wrong though. I've never personally owned a Bernese, so I suggest looking around online. [Here] is a pretty decent site. You could also look at your food brand's website to see if they have any additional information.