Lots of good responses already. In these cases and especially with Biktarvy I still find low level viremia usually comes down to adherence or interactions. Maybe another round of asking about OTCs-taking any creatine, muscle gainers, energy drinks, turmeric and so on?
For adherence, see if you can look at the dispense dates at the pharmacy to see if he's filling on time. Offer a pill organizer to hopefully highlight to patient if he indeed has been missing doses, set a daily phone alarm.Is partner also on Biktarvy, could there be pill sharing? Any GI issues that would impact absorption? Good luck.