[Help] Our adopted dog barks/growls at my partner

My boyfriend and I have been living together for a year (together for 5), and adopted a terrier mix, Buck, together 6 days ago. The night we got him, my boyfriend was at work, but he came home later and we all seemed fine on the couch together. For next couple days, Buck bonded more quickly with me, and seemed timid around bf. We both thought it was because I work less and am in the house more often, so my friend became the only high value treat giver. However, yesterday Buck growled at bf after he got out of the shower. We calmed him down, and tried hard to have my bf give him treats every time buck came near him. This worked for all of last night. However, today he growled on and off at bf for over an hour, even though he was sitting on the couch not moving. What's the best course of action from here? He was not at all aggressive with anyone at adoption, and was fine and friendly when a female friend came over.