Thank you for your question.
may be due to bleeding in the skin which is a common side effect of warfarin
This happens when blood is too thin and it can lead to bleeding from other areas like brain, gut e.t.c.
It is highly recommended that he gets his INR test checked today without delay.
If this turns as normal the we may consider other causes of rash which if diagnosed even after few days may not be as harmful.
So make him see his doctor today.
I hope this helps.