Why do you relish alone time? The answer to that question can perhaps serve as an explanation when you set you're boundary if you haven't already, and you can state that it's nothing personal, as you would do the same with most people i assume.

I also agree with knitchick though, just plainly asking him why he spends so much time at your place and why he's afraid to be alone. Was this rather sudden behaviour on his part? If yes, You could maybe preface by saying that he's been acting unusual lately and you're curious or concerned, and since him being in danger has crossed your mind, that might not be too far from the truth.

Also, besides the fact that he constantly wants to stay at your place and/or spend time with you, is there anything else unusual about his behavior that verges on you being uncomfortable?