You'll know what's going on soon enough.
Your instincts are probably correct.
However "loving and caring" you were on the phone, though, it WAS an interrogation. You wanted him to tell you whether he was drinking or not. You questioned him about his choice of cold medication, told him to "be careful" around his meeting because he was slurring.
Of course, he deflected the accusation. That's normal alcoholic behavior.
Are you going to Al-Anon? If not, I strongly suggest it.
Yes, it's a progressive disease. And people progressively find it harder to "bounce back" from a relapse.