First, a little context on what InmateAid actually is. InmateAid is a service that allows people on the outside to send letters, postcards, photos, and magazines to inmates, and allows inmates to write back through the site. It is not a social network, a dating platform, or a way for inmates to reach out to strangers. The inmate himself cannot initiate contact through InmateAid. Someone on the outside has to send something first.
So the more useful question is not whether he signed up, but who on the outside is using the service to send him things. If there is someone you do not know about, sending him mail, that is the conversation to have with him directly.
That said, the discomfort you are feeling is worth paying attention to. A relationship where something feels hidden or where communication is not open tends not to improve over time, especially with the pressures of incarceration layered on top. The distance and the isolation of a prison sentence put enormous strain on trust, and if the foundation of trust was already shaky, that strain tends to make things worse.
The honest advice is the same whether it involves InmateAid or anything else: if you cannot have an open, direct conversation with your boyfriend about who he is in contact with and why, that is the problem worth addressing. The platform is just the surface.