May be my logic is wrong..but I personally think that since budget set is the set of possible points given his income in this case his budget line is given by M=px..but if he consumes x he has to necessarily consume a*x amount of smoke, so he cannot choose any of the combinations, he can only choose from those combination where s=a*x holds as well as p*x<=M holds..all the points where s<a*x and s>a*x he cannot choose..his only option is to choose from s=a*x till the point where the two lines intersect.
"I don't ride side-saddle. I'm as straight as a submarine"