We rarely have sex

So my boyfriend (25) and I(23) are together for around a year and living together. In the beginning we had sex quite often, which I think is normal when you just starting. But nowadays we just have sex every 2 weeks or every 3 weeks. I already talked to him about it and asked if he might be too bored and want to try something new but he always refuses. Once I even tried showering with him but he didn’t want me to shower with him, he just wanted to be done fast. He always says he’s tired or sleeping early and if he’s not home he’s at his friends place “chilling.” (4 to 5 evenings/week). I don’t suspect him cheating and I trust him, but I just can’t figure out what the problem is. We don’t even make out, we just give each other a peck in the morning and in the night. I feel really frustrated and don’t know what to do, as talking to him doesn’t seem to help.. Does anyone have some ideas?