Parsing "他是…" in this short sentence
This sentence is quite oral,
虽然我觉得他是想象不出来的就是了 == 虽然我觉得他想象不出来
That is: 


  Though, I think he can't imagine (it)


As for Though, I think it is unimaginable for him, you should say:


  虽然我觉得这对他来说是不可想象的


And Though, I think I can't imagine him (like that/doing that etc) should be:


  虽然我觉得我无法想象他（是那样的/做了那件事）