Most likely a malevolent spirit entity, what the modern world would miscall a 'demon'.
Astral travel is appears to be the new fad in town. On another forum I read a most interesting account where the poster believed that they had opened themselves up to the acceptance of knowledge from the other side of the veil. Sadly, her account reads like an account of demonic possession.
Astral travel was considered a risky road to travel - Back in my youth we did a lot of protection spells and sought to travel either in groups or with our spirit guide/animal. Mind I was par of a coven for several years, and we took great pains to travel with half the coven in spirit, the other half guarding the physical. We also cast plenty of protections for both spirit and body prior to travel. Back then we took such very seriously.
Today it seems like many people just decide to throw themselves out there without any regard of the risks. And there are plenty of risks, as you and your BF are discovering.
Humans reside in protective shells (bodies) for a reason. The spirit of humans lacks important information, lacks experience, lacks the ability to ward off (fight) the malevolent entities. The process of becoming human, the birth, life and death cycle means that we are born with amnesia, separated from our spiritual life and experiences so we can learn important lessons as a child learns, fresh, new without our old preconceived and often stymieing notions about everything.
I strongly urge you and your BF to learn how to create protective spells. One popular long term protection which as stood the test of time and goes back a long time in history, is the witch(es) bottle. Many sites cover how to make one.
Preparing for travel should also have you laying guards and wards, or protection spells for the duration of the journey. such as lighting a white candle addressed to represent yourself surrounded by a circle of salt.
If you all follow a tradition that includes something along the line of spirit guide, or spirit animal, I would recommend that you travel with your guide/animal. Animal Familiars make for great pets... Understand that you don't necessarily need a cat, any life form that humans can bond with in a social way can usually be trained in the path and become more than the sum of its parts - become a familiar. Traveling with familiar is very safe for single practitioners, if they have taken other precautions as well.
Understand that all magic, all crafting of spells, all sorts of spirit activities shine out in the nether regions into what many call the astral plane. That leap from body to other realm sends out a flash of 'light' - a signal that draws the attention of all things... Its sort of like screaming in a library... Everyone notices, only few will actually ask 'whats wrong'.
Many things live 'out there'. Many are not nice. Some are down right malevolent with dark intent for any would be traveler.
And then there is the dark aspect that humans, those who live in this physical realm and travel the byways of the other realm are not all nice and basically harmless.
Magic tends to lead to empowering individuals, unfortunately power corrupts, and yes absolute power corrupts absolutely.
The fact that your BF had a difficult time reentering his body without doing so naturally (instinctively) is a troubling sign. It is the instinctual self survival aspect of humanity that leads to instant rejoining with one's body at the first sign of trouble. Your BF didn't react that way.
Either he has a weak or impaired instinct, or he is already being controlled by an outside influence that can stymie his own natural instinct to return to 'home' when there is danger.