Why is communication between me and my boyfriend so awkward ? His Mercurys in Pisces Mines in Libra?

That 12th house placement is probably where the discomfort and off timing is coming from. Also check if one of you is Merc retrograde..that indicates pausing and restarting. Basically, you are intellect and he is emotion. Because you lack emotion (from his perspective) when you speak, he finds you insincere. You probably find him melodramatic, and he probably is. You can fix this by using some basic communication techniques...Touch his hand lightly and lean in to him when you talk to him. Body language is very important with Merc. in Pisces...they need a little drama in order to interpret the intent precisely. Also, make sure you keep good eye contact; lower and deepen your voice just a little when you are explaining things to him. For some reason these things seem to indicate sincerity to people like that. I have used these techniques on my Mercury in Pisces daughter and I've found them helpful.