J.S.
Have you asked your son's neurologist about a 24-hour medication? They're not actually/strictly 24 hours. It depends on how the body metabolizes the medication and what the half-life is. If you don't want him to take meds twice a day, there's a longer acting formula for a few different medications.
For instance, my eldest daughter takes Concerta for her ADHD. She takes the long acting formula but she can't take it after 10:30 AM or she needs a dose of melatonin that night. If she takes it at 7:30 AM she feels it wearing off around 6:30-7:00 PM which means she has to make sure her homework is finished by then.
You have to remember that these are controlled substances, and they're serious medications. The medications MUST be more for the child's benefit than for ours. You MUST talk about this with your child's doctor. If you don't have a pediatric neurologist that specializes in ADHD, ADD, and other neurological behavioral disorders then you should get one. They know these medications inside and out and will insist on frequent check-ups where they give the child a quick physical every 1-to-3 months to make sure the medication and dosages are appropriate. They will be a hands on doctor. You don't want a doctor that will simply prescribe something because you requested it.