Hello Sheraz, it's terrific you have joined the site and feel sorry for what's happening because there are people who work in FIFO who also are having problems, although the finances may be good, it can be very awkward when a situation can't be resolved when you are so far apart.
I am very sorry his friend passed away and with being away from you makes it difficult to get the comfort and soothing he's looking for, and may increase his alcohol intake as a result as he's a high functioning alcoholic and certainly increase how he is feeling.
I don't think he wants to break up with you, that could just be a reaction from how he is feeling and feeling OK in the morning is not unusual, although I'm not a doctor,but know from myself, it's only as the day progresses your mood changes, because you start to think of what's actually happening.
FIFO isn't a convenient occupation, yes the money is good, but personally, it makes the person left behind to carry the majority of the weight which the person away doesn't comprehend, especially if you have children.
You do need to look after yourself, that's very important and believing he is in contact with other women is what this illness is creating and besides, he may still have similar problems as now but talk about this with a counsellor.
Is there any chance he has had a thought about his drinking, just asking.