The value of such a letter will depend upon:
1) the context in which you know him (eg "dated my daughter," "was my next door neighbor", or "I've known him since he was a baby and dandled him on my knee" = zero usefulness vs he can attest to your academic prowess in a class he taught, you TAed one of his labs and performed brilliantly, plus you engaged in a research activity under his supervision, showing excellent problem-solving skills, critical thinking, and creative input = application gold) ,
2) the personal characteristics he can attest that you have which are valuable in a physician, and
3) whether his letter gives a good explanation for any weaknesses in your application that you can't overcome.
A great letter is worth even more if the dean at the other school knows him personally.