It sounds as if you’re being honest with him, and as long as his girlfriend knows about the intensity of the relationship between the two of you I think it is worthwhile experiment. The more important thing is for you to realize that the chances of this going well are slim. There are many reasons for this, not the least of which is the potential for jealousy on someone’s part. I think it would be important for you to honor your own feelings with this.
It does sound to me like you have strong feelings for him, they are reciprocated by him, but distance has kept these feelings in check. When long-distance relationships become powerful, close relationships, the dynamics change and feelings often change with them. I think if they are going to come live with you that you do it with an experimental attitude — perhaps a one-month trial to see if it’s all going in the right direction. Without a trial period it may be too much emphasis on having to have it work.
Wishing you patience and peace,