I hate to break it to you, but you may need to come down hard on him. Now DON'T take my advice as the only solution because I COULD be wrong and it can be detrimental if i'm wrong. Maybe he has self esteem issues, inferiority issues. If so, you may need to half the heat on my personal solution below. Again, DON'T follow this if wisdom is telling you so because I don't know what he's going through.
Usually when someone does that to me, I try to be patient, but I think you've been patient with him long enough and you may need to lay it straight to him that he's a complainer. You know what God did to the hebrews in the desert when they complained?
Well, to keep it short and simple, God killed them.
Okay, I realise this isn't easy and that analogy isn't helping.
A technique a leader did on me once was just to speak gently but firmly. Tell him the truth, he's a major complainer and he needs to get his act together and
grow up. Tell him not to waste your time about such things anymore because its just not realistic or reasonable.
If you have to, just hang up on him.
I'm sorry if that will sound rude, but if someone did that to me, I'm not going to let people like that steal my day because someone made a booboo on his knee.
I mean, do what you can to get it through to him. This has gone on long enough. But again, use wisdom, don't be stupid and follow my word like it's some holy commandment. This is just leadership advice.