My boyfriend is bisexual

Hello, I am a straight woman, my boyfriend is bisexual and I am a little confused. First of all, I don’t mean for anything I say to offend anyone, I’m very new to all this, so don’t really understand the right terminology to use, so please bare with me?
So, my boyfriend is bisexual,we have been together for just over a year now, and I have known the whole time that he is bisexual. We have a bit of a long distance relationship. He tends towards men, recently we have had a few issues sexually, eapecially when we have been apart for a while and he has been pleasuring himself thinking about men, is this normal? This is the longest he has not been with a man, so it’s bound to be odd for him. We are incredibly close, I only want what is best for him. Basically, I think my question is, how can I help? It’s frustrating him/making him feel guilty for thinking about men which I think is putting pressure on him sexually when we are together, is there anything I can do?