Is it me or is he cheating?

I have been in my relationship for 17 years. it has always been loving, the sex has been phenomenal. My SO has recently started travelling for work. At first i received text messages of how much being away from me was killing him, how he needed to hold me, etc. A month out I was catching him checking my phone, stalking my Facebook. His behavior became erratic. He started putting me down in front of other people. This last trip home he tells me he wants to join a Gym. I agree that it sounds fantastic and pursue researching Gyms. He tells me that he said that HE wants to join the Gym not us together. He started drinking heavily while out of town and being less available for me, sometimes not calling for a few days. Sexually he flirts with me but when it is time to do the deed he is tired and needs sleep. He allows me to blow him but if I attempt to use that erection he is suddenly limp. He is not ejaculating when he reaches orgasm by oral stimulation. He is moody and quiet unless other people are around and suddenly he gets off on trying to humiliate me in public. Is he going through a change? Or is he no longer into me?