Well firstly after the first few arguments I just told him that I was going to be creating one to see if I can, I said that I didn't really believe it (this was all lies btw but the best chance of getting him slightly on board) and that I was doing it as a test to see if the mind was really capable. That already put him on the side of science rather than mental illness but he still wasn't happy. That's when I just started introducing the idea slowly and rather off hand "Oh I felt a weird response today" and thats all I said and changed the subject, stopped it being a full blown argument and I just got dirty looks instead! It got him thinking about it without feeling pressured and getting all worked up.
For explaining it more scientifically, I have no idea what I said to him was remotely right but I do remember reading someone elses theory which helped me (Can't find it). I guess it's not the best to lie but it's helped him understand and be more open to it. I told him that you have to imagine that the brain is always making new connections, whenever you learn something new more neurons are becoming attached (I'm no scientist, I haven't the foggyest if this is what happens but here is a site that kinda says what I was trying too http://science.howstuffworks.com/life/inside-the-mind/human-brain/10-brain-myths3.htm). So I told him that when creating a tulpa the neurons and synapses that are being created, eventually through the belief and learning, take on their own and become separate. It's when the tulpa learns something, it is their neurons that grow.
This is where I got kind of confused but my bf got quite excited and explained it back to me, he said it was kind of like if you imagine a stem and from the stem there are our neurons of our consciousness growing, learning and changing and then from the same stem further up, there is the tulpas. So two separate consciousness' linked to the same "stem". It's still us but the brain has learned they are separate.
Again I'm going to say that I have no idea if this is how the brain works and I don't know if this is true BUT it helped my bf understand a little bit more. If anyone who is more articulate and science minded wants to change that around to make more sense and more founded in facts rather than me making stuff up, be my guest!