The only thing I can think of for him to be accepted into the Titans is if time and time again he proves loyalty to them, but that would take time and a few arcs. You might just have to plan on him being a villain for a while until certain opportunities come up that tests his allegiance.

Also it might help his chances if he decided to wear something different when entering the Titans Tower. Something without affiliation. Edits happen, I had to change Rose's uniform to fit the particular situation.