I can't imagine what this substance is, if it is constantly there despite bathing him? -unless it is coming from his tongue, and his mouth if he licks himself?
When you bathe him, are you certain you got the substance completely off in the first place?
Could it be something toxic such as some kind of motor oil....etc? Or did he roll in anything, which has been impossible to wash off thoroughly?
Greases are notoriously difficult to wash off.
There is some information about involuntary muscle tremors here:
http://www.petmd.com/dog/conditions/neurological/c_dg_tremors
My feeling is it might be a good idea to ask the vet about both these issues.