If you explain to him very clearly what is going on, and answer all of his questions, to make the truth as palpable as possible he will relent. The truth is, I presume that you cannot be together no matter what happens, and it just could not have been any other way. If you get him to see that this is the case, and why this is so, you should be set. So, you will have to take care of his intellectual needs there to get what you want out of this situation. Clarity and logical consistency is the primary token of INTP behavior, they tend to apply this to everything they do, your romantic relationship is likely no exception.
So, all in all, make sure that the picture makes sense to him, that'd be half the battle if not more. At that point he may have some emotional bruises, but he will not dwell on those sentiments like Feelers often do. As soon as he entirely comes to terms with the situation on the intellectually it will be only a matter of time for him to do so on the emotionally.
