Do I ask him if he is seeing someone else if are not exclusive?

I have been dating someone for six months. He is a merchant marine and has been out to sea for the past three months. We continued talking through email, text, and telephone the time he was away. We had a talk before he got back about what we were and I asked if he saw this going anywhere? He said because of his career and how much he has on his plate he doesn't want to get into anything serious. I am the sweetest girl he has ever met and still wanted to continue dating when he returned.

Since his return a little over a week ago we have talked almost every day and have seen each other a couple of times. When I spent the night with him last I picked up his phone to see what time it was and noticed a text from a girl he dated right before me and found out he has seen her since his return as well. It was completely accidental that I found this out and I know we are not exclusive but he is here for only another couple of months and then back out to sea.

Do I confront him about seeing someone else or just take it for what it is because we are not bf/gf? Or do I just back off and not respond when he calls/texts and give him the hint that something is wrong. Ugh! Confused!
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Thank you for all of your responses. I am still unsure of what I want to do. I am taking a step back and not making myself as available as before. I failed to mention he is moving back to his home state of FL in a couple months
Do I ask him if he is seeing someone else if are not exclusive?
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