The following is from series of questions answered by Shekhar Bodhakar     

If your name is XYZ and Indians want to praise you by hailing your name, they will say, “JAI XYZ” It’s like saying “Hail XYZ”.

Similarly,

If your name is BHEEM, they will say JAI BHEEM.

And

If your name is BHIM, they will say “JAI BHIM”

I assume the person you are referring to is Dr Bhimrao Ambedkar and not Bheemrao Ambedkar. So “JAI BHIM” (Bhim being short of Bhimrao) would be the correct spelling.