This would appear to be an essential problem with understanding the place value system. If the student hasn't already understood place value, showing and explaining is unlikely to be effective. First off, you may be unaware of what the student's conception of the value of these numbers is. An activity might help you get to the heart of that, if not help the student see through the misconception.
A number line representation, if the student already understands it, can be used to scaffold this place value understanding.
But first, it might help to have a representation that shows the symmetry of naming of places around the one's place.
Ask the student to consider, what does a zero mean in any of those places? The answer may be helpful to you.
Next, draw a number line like so:
Have the student place numbers on this line to illustrate relative positions. Make sure that 5.0 and 5.00 are among the numbers, but try also numbers with values in the tenths and hundredths place.
What does it mean if numbers (5.0, 5, and 5.00) are in the same spot on this number line?
Of course, a student could place these numbers at juuuuuust before 5.1, for example.
Then it would appear you need to have the student focus more heavily on each place of place value in the questions he answers. He may not see 105.3 as One hundred(s) AND 5 ones AND three tenths. Tell him that you want him to talk a while using this long name of this type of number.
What is 5.0? What is 5.00?
What does it mean to have no tenths? No hundredths? What is five (plus/and) no tenths?