RealDarkSeraph
Thé problèm is that they would never accept him as custodes . liké to become a custodes your record must bé clean and you must bé a High born from Terra. if you dont fill those criteria, There is zéro chance for him to become one. I would however think he would be more suited for the Officio Assasinorum or the Astra militarum. or hé could just become a Rogue Trader.
The Adeptus Custodes are the best choice due to the fact that they are the most superior Space Marines, mainly because they are made from the Emperor's own DNA, making them True Demigods compared to the average Space Marines. They are even superior to the Primarchs created by the Emperor. The only downside is that they are restricted to guarding the Corpse of the Emperor rather than fighting anywhere, meaning you'll probably have to make him a unique position in the imperium as a whole so that he can be involved rather than remain at the Emperor's throne room.