Working with a dog who is not motivated by anything and very complex

I was wondering if anyone here has any advice on working with a very complex unmotivated dog. I am not calling him unmotivated in general just unmotivated to work for anything. Gonzo is my 5 year old Belgian Sheepdog Border Collie mix that I am training at my vet tech college. I fostered him for 2 weeks and am most likely bringing him home(this decision is still up in the air as I live at home with my mom still who has a final say on all this as it is not just me in the house).

He is not treat motivated or toy motivated. We have tried every type of treat and toy out there and he just doesn't want anything to do with them. He will work for praise but not all the time. He is best described as an onion that has many layers to him and we are still trying to figure this guy out.

If I end up adopting this guy I will be seeking the help of a behaviorist as he is a complex dog. Has anyone ever worked with a dog like this before? One where no treats or toys will motivate them, they are very independent, complex, shy, skittish, unsure, unconfident and have any tips? I can provide more information if needed as well.

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Update:

I am not taking him. He is going to go back to a shelter and most likely be put down as no one else wants him. My mom(I live with her so she has a final say) flat out refuses to take him. It sucks but nothing I can do about it.