I doubt that the T is good. The T suggested complete transparency, so the T is treating this issue as a behavioral modification - transparency is used in an attempt to change the pattern of behavior. I do not see any effort on the part of the T to get to the root of the confusion and surrounding issues. Maybe a longer term T is needed, as there is no quick fix to this.

You are right to suspect that citing abuse and addiction may be cop-out attempts; of course, they may be valid, too, but you never know. I think you should shut off physical contact only if you suspect that he might have been sexually active with someone other than you and might have become infected and thus a danger to you. Otherwise, if you feel drawn to him and he, to you, there is no reason to shut off physical contact. You have to be reasonable and avoid knee-jerk reactions.

I would not spy on him - just tell him that he has three months to figure it out, and during that time he can message/text etc. however much he wants while also going to see a T in order to collect information that would help him, jointly with the T, get to the root of the problem. Maybe have him keep a spreadsheet where he lists how he feels before he sends a picture of say, him in your bra, the type of picture sent, and, the reaction after he sent it - his feelings right after, an hour and three hours after. Make him be a lab coat researcher, etc., trying to figure out what is going on. If you personally want transparency (rather than the T), you can ask for access to the spreadsheet.

Pictures taken on modern devices, such as smartphones, are usually dated and timestamped. It should not be difficult to date the picture of him in your bra. That he tried to say that it was sent months before it actually was sent shows that either your bf is desperate, or that he thinks that you are technologically naive, or both.