It sounds like your boyfriend lacks belief and trust in himself. This was probably triggered by a specific incident that he chose to bury deep below the surface.
I would suggest that he sees a hypnotherapist who can help him discover what has caused the insecurity and fear. It may have to do with his mother or another significant female in his past.
I am saying that because you have noticed that the more you and he get attached, the worse the situation is getting. There may be an underlying fear that you will leave him, and that may be the reason why he is fixating his insecurity on you.
If he does not get professional help in finding the underlying cause of his behavior, you will find that you will get quite exhausted when you are with him. That will be because he is getting energy from you rather than generating his own energy, and that is never a healthy situation.
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