Love and technology

you should talk with him about it and think together how you will be able to fill more comfortable about it. I hope he is the kind og guy that could understand your feeling and let you feel better.

For yourself, you surely knows that if he realy likes to be have other relations, nothing will prevent him. He can always open another messanger you are not in it etc. So...you have to learn to leave with it (its not only technology- a man can go to a meeting at work and meet a really nice girl.....)

Like Dafna said, I don't think this is about technology, I think this is about trust - a lot of people use Messenger at work or in other situations, to talk to friends or co-worker or to get to know new people. I did it a lot when I was in a relationship and it was really with no romantic or sexual intent, it was just a way to pass time at work...

If you have a bad feeling about it, you should talk to your boyfriend about it. From what you are saying there's little to be suspicious about, however, I don't think that keeping your fears in will benefit to your relationship.  

Relax - just send him an SMS

Being connected on one chat does not mean that you necessary are ready to answer or spending time on it.