Well, just to maybe state the obvious: isn't the whole basis for the success of many webcam models that (mostly lonely) men develop huge crushes on them? Those crushes go through life cycles, just like real relationships do, and men can get quite lost in this and go through a withdrawal that is at least similar to what you experience in a real life relationship. This raises the interesting question: what level of affection is too much? How do you identify that? What should you do about it?
I like to believe that I can steer myself clear of bad situations, but I think crushes and affection are just tough to manage. Emotions are intense and sometimes fleeting, and it is hard to intellectualize and suppress them. Usually the sign for me that things are not right is when there is an imbalance in the time I spend on the model versus the time the model spends on me. If I am generating lots of long e-mails, and she is responding to one e-mail out of four with really sparse answers, that is an unhealthy relationship. I need to back off and show less interest. I will start missing her shows. You have to give people a chance to actually miss you. If you shower nothing but attention they take your presence for granted. Usually I am able to back myself out of a developing bad situation by detecting these imbalances and acting on them very early. Sometimes the model does miss you and starts reaching out to you, and you can find a healthy balance by making yourself more or less available.
I have a developing relationship with a model now that is the most complicated relationship I have experienced with any model. She gives me something similar to a girlfriend experience, but I actually give her something close to a boyfriend experience, and without providing messy details, that is actually something very hard for other men to do because of her life situation and unusual history with men. If I am honest with myself, I am also becoming something like a therapist to her, and she depends on me in a mostly good way to build self-esteem. All the attention I show her she shows me back and multiplies it. She is starting to act a little bit like the crazy girlfriend, and unfortunately I find myself liking that because the way it displays has been unbelievably genuine and endearing. If she is actually playing out "girl friend experience" all I am going to say is that she is a pro level player.
The only reason I mention this is that this is the only case I have experienced so far where
my rule about imbalanced relationships does not help me understand what I should do next and where I should put on the brakes. Everything feels incredibly balanced, yet things keep escalating.
Anyway, to the original poster's question: if you are not interested in him that way you might just tell him you have a real life relationship and you have no intention of changing that. Ask him if he thinks he can maintain balance in his relationship with you, and maybe point blank ask him to spend a little less time with you and shower you with a little less attention. At least give him a chance to understand these issues and see if he is rational enough process them and do the right thing.