He's not giving me as much attention...is this a bad sign?

my boyfriend and i are in love and weve been going out for nearly a year now.we're so attentive and loving to each other when were together, but when we have to spend a few days apart here and there ; he never really seems to put any effort into contacting me.especially in the evenings which is making me suspicious.he used to be very attentive in the beginning always sending me good night texts and replying to my messages but recently he has stopped doing so.i miss him so much when were apart,but this leads me to believe im not really on his mind when im not with him.It just makes me wonder what he is doing with his time.last weekend i went out with sum of his guy mates and my bf and they were talkin to us about one of ther friends who always cheats on his girlfriend.my boyfriend seemed to think this was funny that his friend was unfaithful to his girlfriend and viewed it as trivial.this got me thinking if he has the same kind disrespectful attitude towards our relationship and alarm bells started ringing.when i called him up on we he was laughing he told me that he didnt know that would upset me and that it doesnt have any implications on us as a couple.this insight mixed with the lack of communication recently when we are apart is making me distrustful. am i overeacting or should i be cautious? or is this lack of txting, calling due to the fact that he is just getting comfortable in the relationship and doesnt feel the need to communicate with me as much?your views would be appreciated thank you.x