Hi Mom,
This is a great question to ask your son's doctor. I am not a physician, so can only tell you what I've observed in others and what they've told me.
Some people do develop a tolerance to stimulants over time; others do not and do quite well on the same dosage for many many years. If you stop giving you son his meds on the weekend (and please discuss this with his doctor before considering doing that), I don't think that a lower dosage during the week would solve the problem. Stimulants do not stay in the body for long, so regardless of whether he's taking it on the weekend or not, he'll most likely go back to ‘status quo" during his school week. In other words, I don't think he'll need any less just because he didn't take it during the weekend. Nor do I think that a lower dose would last longer.
Best to discuss these concerns with his doctor.
Terry