It appears that rivaroxaban can cause raised blood sugar levels but it is very uncommon. (0.1% to 1%)
https://www.drugs.com/sfx/rivaroxaban-side-effects.html
Increasing Metformin won't have had any effect at all. It doesn't work that way. It won't have either increased his levels or prevent an increase. It helps with insulin resistance and helps reduce the amount of glucose the liver produces naturally, but only to a limited extent. It doesn't help at all with post meal spikes. It is mainly an appetite suppressant.
Which reading was 16.4? Was this a fasting level or a post meal level? What did it used to be before the rivaroxaban?
Does he test his own blood sugars before and after eating, and what type of foods does he eat?