Does your boss want a sign forbidding the spitting of the tobacco, or forbidding the chewing of it in there in the first place? Around here, smoking in public buildings is absolutely forbidden and I would assume the use of other tobacco products is also. If this is the case, or you just don't want him chewing it in there, you could write something like "Please do not smoke or use chewing tobacco in the restroom" and cite local laws if they happen to apply.