It seems quite strange that he is acting this way, have u tried asking him why he constantly texts you, and if he states something around the borderline "I fancy you" , you can tell him "you have a girlfriend and its wrong", but i wouldnt get involved with a man who already has a girlfriend. If he is still with his gf and texts you , and u get together, god knows he could do that to another girl whilst being with you. Get things cleared with him first and tell him to be honest and take things from there
good luck and keep us updated
Desire xxxx