The lack of interest in sex is a symptom, especially when he dismisses it when the conversation ends in an argument.
Have some patience, back off and observe. There could be other things that may have changed. Like I always say, go by their actions, not what they tell you.
I have a gut feeling he's not being honest. I can't tell you what, if there is something serious going on, but it does need investigating. I am not an avocate to snooping, but if the behavior is changing, then it's worth looking at whome he maybe be chatting with, or there's a new coworker, old flame, someone that has entered his life.
Does it seem his attention is focused somewhere else? He hides his phone, works late more often, etc. What else has changed?