minesawine that's a difficult one.
Has your DS got a conditional or unconditional offer? If the former, then strongly remind him that five sessions of tutor input on exam technique could make the difference twixt getting the grades or not.
You could compromise - three sessions rather than five (and potentially up them to 3 hours apiece...)?
You could say that as an adult, the decision is his, but maybe go through the pros and cons of continuing the tutoring with him, hoping that the positives will outweigh the negatives. If he comes to the realisation himself, without being 'told what's for the best' he is more likely to accept it.